The effect of a mixture of naturally occurring aversectin C and avermectin B(1) on the growth of ascites and solid experimental tumors of mice was studied. It was shown for the first time that avermectins possess a pronounced antitumor action. When added at nontoxic doses, they significantly suppressed the growth of ascites Ehrlich carcinoma and P388 lympholeukemia and solid Ehrlich and 755 carcinomata. With some administration regimens, avermectins suppressed the tumor growth by 70-80%. Avermectins were most effective when injected intraperitoneally. It was also shown that avermectins enhanced the vincristine-induced suppression of the growth of Ehrlich carcinoma, melanoma B16, and P388 lympholeukemia. Avermectins produced this effect only when injected after vincristine.