The inhibitory effect of pheophorbide on the mutagenicity of benzo (a) pyrene with or without light irradiation was studied. Without light irradiation, 250 μg pheophorbide reduced the mutagenicity of benzo (a) pyrene to about 90%. This inhibitory effect was the same or stronger than that of hemin. On the other hand, pheophorbide was rendered mutagenic by exposure to light, and the mutagenicity was 40% that of 5 μg benzo (a) pyrene. When 5 μg benzo (a) pyrene coexisted with 250 μg pheophorbide under light irradiation, the mutagenicity of benzo (a) pyrene was apparently reduced to 30% because of the antimutagenic effect of pheophorbide.