Clofibrate as a sole agent was found to have statistically perceptible antitumor effects against the in vivo growth of Walker 256 intramuscular carcinosarcoma in rats. Clofibrate was also found to potentiate the antitumor effect of hydrazine sulfate, cytotoxic drugs and the combination of hydrazine sulfate + cytotoxic drug. The results suggest that substances which might directly or indirectly interfere with host lipogenesis and/or lipolysis, may also have adjunctal antitumor potential.