Rate division of human lymphocytes was studied in 85 healthy siblings and 142 normal non-related individuals using sister chromatid differential staining in standard conditions of cell culturing. It was shown that proliferation of male lymphocytes exceeds that of female cells. This sexual dimorphism does not depend on the time of fixation of cell cultures and probably is conditioned by differential chromosomal constitution of the two sexes. The study of sex chromosome mosaics revealed rate modification of cell proliferation in the line: 45,X greater than 46,XY greater than 46,XX. The possibility of influence of heterochromatin and sex chromosomal genes on control of cell division is discussed.