The object of this investigation was the distribution of human periferal blood cultured obtained from different donors with respect to their sensitivity to different concentrations of thiophosphamide (10, 20 and 30 mcg/ml for 1 hour). Two test characteristics were studied, "the proportion of aberrant metaphases" and "the number of chromosome breaks per 100 cells". It is shown that the distribution of individuals does not depend on the mutagen concentration, being normal in all the series of experiments. The same results were obtained when two different statistical methods were used, viz. the method of four moments and the chi-square test. Since the former method is less labour-consuming, the authors recommend it for fitting the normal distribution in cytogenetic investigations. The variances of the test-characteristics studies ("percent aberrant metaphases" and "total number of breaks per 100 cells") increased with the increase of the mean values. Therefore, before the analysis of variance and the regression analysis the cytogenetic data should be preliminary transformed for the purpose of stabilization of variances.