Two types of hydroxy acids, (10E, 12Z)-9-hydrouxy-10,12-octadecadienoic acid [1] and (9Z, 11E)-13-hydroxy-9,11-octadecadienoic acid [2], were obtained as cytotoxic compounds from a water extract of rice bran. The cytotoxicity of acid 1 was stronger than that of acid 2 in vitro, against P388 mouse leukemia cells. Acid 1 slightly inhibited the growth of a Balb/c mouse 3T3 fibroblast cells (BALB/c 3T3), but strongly inhibited that of simian virus 40-transformed 3T3 cells (SV-T2); 1 was therefore, found to have cytotoxicity against cancer cells. On the other hand, 2 inhibited neither normals cells nor transformed cells. Intraperitoneal injection of acid 1 effectively prolonged the survival time of C3H/HeJ mice after intraperitoneal transplantation of MM46 mammary tumor cells. The difference in antitumor activity between 1 and 2 was also observed in vivo.