The established improved 3-(4,5-dimethyl thiazoly)2,5-diphenyl-tetrazolium bromide (MTT) colorimetric assay for measuring the amount and function of living cell by the authors has the characteristics of simplicity, dependability and good repeatability within 3-12 hours. We have used this improved MTT colorimetric assay in measuring natural kill cell (NKC) activity of human peripheral blood. The results show that when the activities of effector cell (EC) and target cell (TC) are > or = 95%, and the density of EC is > or = 3 x 10(2)/well and that of TC > or = 1.6 x 10(2)/well, the number of living cell is positively correlated with the OD570nm volume of purple formazan metabolite of MTT solution (re = 0.94, ri = 0.97, P < 0.001). When the ratio of EC/TC is 40:1, the NKC activity of peripheral blood of health adults is equal to that report by Zhang Chongjie et al. By this MTT colorimetric assay, the NKC activity of peripheral blood of 10 leukemic patients has been detected. The mean +/- s of NKC activity is 34.86 +/- 7.80%, which is postively correlated with the result (35.16 +/- 8.40%) of the inhibition of 3H-TdR incorporation assay performed at the same time (r = 0.876, P < 0.001). MTT colormetric assay for NKC activity avoids getting in touch with isotope. It is a convenient and valuable method.