A test system has been adapted for evaluating the cytotoxicity of human natural killer cells (NK) towards tumor targets K-562 in vitro. The method is based on the capacity of mitochondrial enzymes of viable cells to reduce the yellow soluble salt MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide) to a purple-blue insoluble formasane precipitate which is quantified spectrophotometrically after dissolution in an organic solvent. The amount of MTT-formasane production is directly proportionate to the count of viable cells. The optimal conditions for the cytotoxicity assay were determined: cell count 10 x 10(3) or 20 x 10(3) cells/well for K-562 as targets and 100 or 200 x 10(3) cells/well for peripheral blood mononuclears as effectors at effector/target cell ratios 20:1, 10:1, and 5:1. This colorimetric method has several advantages over radioactive methods: multiwell scanning spectrophotometers permit the processing of numerous samples and are safe. The results of MTT assay and morphological assay for measuring the NK cytotoxicity were in good correlation and were similarly sensitive.