The MTT cell proliferation assay is a gold-standard colorimetric technique for quantifying cell viability and metabolic activity based on mitochondrial dehydrogenase activity. This comprehensive protocol guide details every operating step of the MTT cell proliferation experiment, from cell seeding and reagent preparation to formazan solubilization and absorbance measurement. Follow these standardized procedures to generate reproducible data for drug screening, cytotoxicity evaluation, and proliferation studies.
Technical Overview
The MTT assay is a colorimetric method used to measure cellular metabolic activity as an indicator of cell viability, proliferation, and cytotoxicity. MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) is a yellow, water-soluble tetrazolium salt that penetrates viable cells. Inside metabolically active cells, mitochondrial succinate dehydrogenase and cytoplasmic NAD(P)H-dependent reductases cleave the tetrazolium ring, converting MTT into insoluble purple formazan crystals that accumulate on the cell surface and in intracellular compartments.
Because only living cells with functional mitochondria can catalyze this reduction, the quantity of formazan produced is directly proportional to the number of viable cells. After dissolving the formazan crystals in a solubilization agent such as DMSO or acidified isopropanol, the optical density (OD) of the purple solution is measured spectrophotometrically at a wavelength of 540–570 nm, with 630–690 nm commonly used as a reference wavelength for background correction.
The MTT assay is widely favored for its simplicity, low cost, high throughput compatibility with 96-well microplates, and lack of radioactive materials. However, because the endpoint reflects metabolic activity rather than direct cell counts, cell number interpretation requires that treatment conditions do not independently alter mitochondrial enzyme activity.
Testing Samples
Adherent Tumor Cell Lines
Commonly tested adherent lines include HeLa (cervical carcinoma), MCF-7 and MDA-MB-231 (breast cancer), A549 (lung carcinoma), HepG2 (hepatocellular carcinoma), HCT116 (colon cancer), and PC-12 (pheochromocytoma). Adherent cells are seeded in monolayers and are particularly straightforward to handle because the culture medium can be removed easily before solubilization.
Suspension Cell Lines
Hematopoietic and immune-derived lines such as Jurkat, K562, THP-1, and HL-60 grow in suspension. These cells require centrifugation steps (typically 1,000 × g for 5 minutes) before medium removal to prevent cell loss during the washing and solubilization stages of the protocol.
Primary Cells and Stem Cells
Primary hepatocytes, fibroblasts, endothelial cells, and mesenchymal stem cells can be evaluated with the MTT assay, although seeding densities must be optimized because these cells often exhibit slower growth rates and greater sensitivity to handling stress.
Test Compounds and Treatment Conditions
Samples evaluated in parallel with the assay include small-molecule drugs, natural product extracts, nanoparticles, antibodies, cytokines, and environmental toxicants. Compounds should be dissolved in DMSO or appropriate solvents, with the final solvent concentration kept at or below 0.1% (v/v) to avoid solvent-induced cytotoxicity.
Testing Items
The MTT assay supports the following quantitative endpoints:
- Cell viability percentage relative to untreated control wells
- Cell proliferation rate over defined time intervals (24, 48, and 72 hours)
- Growth inhibition rate of test compounds at graded concentrations
- IC50 determination through dose-response curve fitting
- Cytotoxicity screening of pharmaceutical candidates and biomaterials
- Time-dependent and concentration-dependent drug activity profiling
Testing Methods
Reagents and Equipment
- MTT stock solution: 5 mg/mL in sterile PBS, filtered through a 0.22 µm membrane, protected from light, and stored at −20°C for up to four weeks or at 4°C for one week
- Complete culture medium (e.g., DMEM or RPMI-1640 supplemented with 10% fetal bovine serum)
- Solubilization solution: DMSO, or acidified isopropanol (0.04 M HCl in isopropanol)
- Sterile 96-well flat-bottom microplates with low evaporation lids
- Multichannel pipette, sterile pipette tips, and a plate centrifuge for suspension cells
- Microplate spectrophotometer capable of reading 570 nm with a 630–690 nm reference filter
- CO2 incubator maintained at 37°C with 5% CO2 and saturated humidity
Step-by-Step Operating Procedure
- Step 1 – Cell preparation: Harvest cells in the logarithmic growth phase, count them with a hemocytometer or automated counter, and confirm viability above 90% using trypan blue exclusion.
- Step 2 – Cell seeding: Dilute cells to 5 × 104 to 1 × 105 cells/mL and dispense 100 µL per well into a 96-well plate, targeting 5,000–10,000 cells per well. Seed at least 5–6 replicate wells per condition.
- Step 3 – Plate layout: Include a blank group (medium plus MTT, no cells) for background subtraction, a negative control group (untreated cells), and treatment groups receiving graded compound concentrations, typically 5–8 concentrations in a 2-fold or 3-fold dilution series.
- Step 4 – Pre-incubation: Incubate the seeded plate at 37°C, 5% CO2 for 12–24 hours to allow cell attachment and recovery before drug addition.
- Step 5 – Compound treatment: Add test compounds at predetermined concentrations in a final volume of 100–200 µL per well, keeping solvent concentration constant across all groups. Incubate for the designated exposure period, commonly 24, 48, or 72 hours.
- Step 6 – MTT addition: Add 10–20 µL of 5 mg/mL MTT stock solution directly to each well to achieve a final concentration of 0.5 mg/mL. Avoid introducing bubbles during pipetting.
- Step 7 – Incubation with MTT: Incubate the plate for 3–4 hours at 37°C in the dark until purple formazan crystals become visible under an inverted microscope.
- Step 8 – Medium removal: Carefully aspirate the culture medium from adherent cultures without disturbing the formazan deposit. For suspension cells, centrifuge the plate at 1,000 × g for 5 minutes and then remove the supernatant.
- Step 9 – Formazan solubilization: Add 100–150 µL of DMSO to each well and gently shake or pipette-mix for 10 minutes on an orbital shaker until all crystals are fully dissolved and the solution is homogeneous.
- Step 10 – Absorbance measurement: Read the optical density at 570 nm using a 630–690 nm reference wavelength to correct for plate artifacts and fingerprints.
- Step 11 – Data calculation: Calculate cell viability using the formula: Viability (%) = (OD treatment − OD blank) / (OD control − OD blank) × 100%. Determine IC50 values by nonlinear regression of the dose-response curve.
Quality Control Considerations
Fill unused perimeter wells with sterile PBS or medium to minimize the edge effect caused by evaporation. Verify that control well absorbance values fall within the linear range of the assay, typically an OD570 between 0.5 and 1.5. Include repeat independent experiments (at least three biological replicates) and report results as mean ± standard deviation with appropriate statistical analysis.
Applications
The MTT assay is applied extensively across academic research, pharmaceutical development, and industrial biocompatibility testing:
- High-throughput screening of anti-cancer drug candidates and identification of potent proliferation inhibitors
- Evaluation of chemosensitivity and radiosensitivity profiles of patient-derived tumor cells
- Safety and cytotoxicity assessment of nanoparticles, medical device extracts, and biomaterials
- Bioactivity screening of natural products, herbal extracts, and functional food ingredients
- Verification of gene knockdown or overexpression effects on cell growth
- Assessment of growth factor, cytokine, and antibody effects on primary cell expansion
FAQ
Why must blank wells be included in every plate?
Blank wells containing medium and MTT without cells account for background absorbance from the medium, residual dye, and the plate itself. Subtracting the blank OD value eliminates systematic error and is essential for accurate viability calculations.
What causes low or high absorbance readings in control wells?
Insufficient cell seeding density, excessive cell death, or incomplete formazan dissolution produces low OD values, while over-confluent cultures beyond the linear range produce saturated readings. Optimization of seeding density and incubation time is required for each new cell line.
Why did the formazan crystals fail to dissolve completely?
Incomplete dissolution usually results from insufficient DMSO volume, inadequate mixing time, or residual medium diluting the solvent. Extend shaking to 15 minutes and confirm complete solubilization visually before reading the plate.
Can the MTT reagent be used after freeze-thaw cycles?
Repeated freeze-thaw cycles degrade MTT and increase background signal. Aliquot the stock solution into single-use volumes and discard any reagent that shows discoloration or particulate contamination.
How does the MTT assay differ from CCK-8 or resazurin assays?
The MTT endpoint is lethal to cells because formazan requires a solubilization step, whereas water-soluble dyes such as WST-1, CCK-8, and resazurin permit continuous monitoring of the same culture. MTT remains the most economical and widely validated option for single-endpoint viability measurement.
How is the appropriate cell seeding density determined?
Perform a preliminary growth curve by seeding a density gradient from 1,000 to 20,000 cells per well and measuring OD values at each planned time point. Select a density that keeps the control group within the linear detection range throughout the longest incubation period.