Heterogeneity of cell and clone populations in various histogenesis of rat tumours has been studied by the content of the endogenic SH-groups. The distribution of the ovary tumour cells and their clone lines, as well as lung clones of the sublines of rat rhabdomyosarcoma RA-2 as to the content of endogenic SH-groups is of persistent and monopeak character. Modal classes are well expressed, in several clone lines variability of the cell composition is higher than in the initial population. In this case the distribution of cells as to the SH-group content differs from the normal (asymmetrically). The level of SH-groups in the RA-2 rat subclone in 71 lung clones positively correlates with the level in hypodermic transplants. So, the heterogeneity of the clone populations as to the amount of SH-groups depends on both the genetic interclonal differences and fluctuations of the environmental factors.