The influence of Ukrain (exerting a malignotoxic effect in vivo according to Nowicky J.W. et al ) on the growth of Hepatoma A (HA-1) tumors in mice and cysteine proteinases as markers of tumor growth and malignancy was studied. Ukrain administration (0.5 mg per mice of 20 g, in each of 5 i.p. injections) resulted in a reproducible and significant retardation of HA-1 tumor growth in the liver and a prolongation of lifespan compared with the untreated control. Repeated Ukrain administration to mice increased the number of macrophages in ascites, decreased the number of tumor cells, and concomitantly reduced the increased level of monocytes in peripheral blood. In ascitic cells the specific activity of cathepsin B was higher than in the intact liver and was not influenced by Ukrain; activity of cysteine proteinases studied in ascitic fluid was much higher than that in serum. Tumor growth was followed by a decrease in cathepsin B activity in the liver and serum at day 10. Ukrain administration has a tendency to normalize the disturbances of cathepsin B activity during tumor development. The cathepsin L activity changes in ascitic fluid were more impressive than those of cathepsin B, indicating the special role of this cysteine proteinase in HA-1 tumor growth and invasiveness. The role of cysteine proteinases as markers of tumor malignancy and invasiveness is discussed.