The effect of bovine pancreatic A RNase and bull seminal vesicle AS RNase on proliferation of HeLa cells and LEP cells in vitro was studied. The ribonucleases were used in doses of 0.1, 1.0, 10.0, and 100.0 microgram per ml of medium and 100 000 inoculated cells. Proliferation was evaluated by the growth curves. Both in single and long-term experiments. A RNase in all doses used had no effect on cell proliferation, whereas AS RNase exerted inhibitory activity beginning with the dose of 10 microgram, and affected more markedly the LEP cells (a diploid non-malignant cell line) than the malignant heteroploid HeLa cells. When added to the cell suspension, AS RNase was shown by indirect immunofluorescence to bind to HeLa cell membranes in all phases of growth, whereas AS RNase was found on membranes of LEP cells only when they were derived from the log phase of growth.