Background: MTT assay is a colorimetric test to evaluate cell metabolic activity of living cells via reduction of the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide to insoluble formazan crystals by mitochondrial activities. The reduction of the tetrazolium dye is thought to occur by NADPH-dependent oxidoreductase enzymes in the cell cytosol. The MTT test is used to measure the cytotoxicity or cytostatic activity of generally plant and chemical compounds and toxic materials. In this study, it was aimed to monitor the uniformity of formazan formation at equal time intervals by visualizing the reduction of tetrazolium salts in cells. Materials and Methods: In the study, K562 cells were used to observe the reduction of tetrazolium salts to MTT formazan crystals in cells. K562 cells were seeded in culture plate under sterile conditions. After adding 10 µL of 5 mg/mL MTT solution to the culture plate, the cells were incubated for 4 h at 37 °C in a humidified environment with 5% CO2. During the culture process, the cells were imaged at 15 minuteintervals for 4 hours. Results: The behavior of viable and non-viable cells against MTT and the process of converting MTT to MTT formazan crystal by living cells were clearly monitored. Conclusions: Visual analysis of MTT reduction directly from the incubator with image recordings at equal time intervals showed the perfect homogeneity of MTT degradation of the cells over time. With our study, we can state that the MTT test is an ideal test method for cytotoxicity research.