The MTT (3-[4,5-dimethylthiazol-2yl]-2,5-diphenyl tetrazolium bromide) growth assay developed by Mosmann (1 ) offers a simple, rapid, and precise measurement of cell viability and proliferation of adherent cell lines (2 ). The value of this assay is in the screening of large numbers of samples. The MTT assay, a quantitative colorimetric assay is based on the living cell’s ability to reduce the tetrazolium salt MTT, a pale yellow substrate to a dark-blue formazan product. The mitochondrial succinate-dehydrogenases (3 ) of viable cells cleave the tetrazolium ring in active mitochondria into formazan crystals. The crystals can be dissolved in acid isopropyl alcohol, mineral oil (4 ), or dimethyl sulfoxide (DMSO) (5 ). The resulting blue solution can be measured semiautomatically using a scanning multiwell spectrophotometer. Our laboratory has successfully applied the MTT-based growth assay with some modifications (6 ) to investigate the growth effects of human cytokines on HOC cell lines (7 ), but we have to keep its limitations and pitfalls in mind (see Note 1 ). For use in tests of floating cell lines, the MTT assay may be less optimal. Using this assay for screening of primary tumor samples may produce limited results, because cell contaminants may result in high-background values (4 ). However, accepting the limitations of the MTT assay, the optimum assay conditions have to be selected and adapted to the cell lines that are under investigation. The MTT-based growth assay, as described in this chapter, is a reliable and sensitive test for the determination of cell growth of human ovarian-carcinoma cells.