We studied the effect of the anticancer drug Ukrain on Lewis carcinoma in C57BL/6 mice. A marked progressive anticancer and antimetastatic effect was observed. On day 33 of tumor development, the tumor growth inhibition and metastases inhibition index were 71.5% and 73.1%, respectively. The antimetastatic effect of the drug manifested as a decrease in both number and volume of lung metastases. This finding demonstrates that treatment with Ukrain in Lewis carcinoma-bearing mice not only prevents the formation of new metastases but also inhibits the growth of existing ones. In addition to possessing anticancer and antimetastatic effects, Ukrain has been demonstrated to affect parameters characterizing the state of the immune system. This effect manifested as an increase in the endocrine function of the thymus (a central organ of the immune system), an increase in serum interferon, adhesion of peritoneal macrophages and formation of antibodies against thymus-dependent antigen by spleen plasma cells. Together with intensification of functional activity of the immune system, Ukrain increased the number of lymphoid cells and monocytes in peripheral blood. In addition to the increase in lymphocytes, the number of large granular lymphocytes also increased, i.e., cells possessing natural cytotoxic activity. The results obtained suggest that the antitumor, and especially the antimetastatic, effect of Ukrain is due to its immune-modulating properties, specifically, activation of antitumor resistance concerned with release of interferon and thymus humoral factors. Natural killer (NK) activity is one part of this resistance. NK cells are able to recognize and eliminate from the circulation cells with the potential to form metastases.