Bull seminal vesicle ribonuclease, when added in vitro to the suspension of ascitic leukaemic cells EL-4 and BP-8 for 2 hours at 37 degrees C and then transplanted to the abdominal cavities of mice, inhibitied the proliferation of these cells in vivo. When the temperature of the medium was decreased to 25 degrees C, AS RNase was bound to the leukaemic cell surfaces, but it exerted no inhibitory effect on BP-8 cells and showed a partial effect on EL-4 cells even after 24 hours. AS RNase administered to mice with leukaemic cells in diffusion chambers significantly decreased the number of cells inside the chambers as compared to the controls. The number of ascites cells was also significantly reduced after the injection of AS RNase for 1 week into mice with leukaemic cells implanted in the abdominal cavities. The reduction of cells in ascitic fluid did not cause a significant prolongation of survival of experimental mice. Only mice with transplanted BP-8 cells had significantly less cells in ascitic fluid and their survival was prolonged to 48 days in comparison with 25 days' survival in control mice after administration of pure AS RNase. In addition, AS RNase depressed the growth of solid tumours BP-8 and EL-4. AS RNase did not change the weight of body organs except the testes. Absorption experiments showed increased absorption of AS RNase by leukaemic cells BP-8 and EL-4 and by testes and spleen homogenates as compared with other body organs.