We investigated the effect of quercetin on 3-methylcholanthrene (MCA) carcinogenesis in C57Bl/6 mice. We found that quercetin itself was not carcinogenic when administered i.m., even at a dose of 20 mg, throughout an observation period of 420 days. An i.m. administration of various doses of quercetin admixed with 1.0 mg of MCA, however, significantly shortened the mean latency periods for the development of local primary tumors compared with those of the group which had been given MCA alone. The shortening of latency periods was also found in the experiments using 0.1 mg of MCA after the administration of quercetin either admixed with MCA or fed with a diet containing it. Moreover, lung metastasis increased in mice given the mixture of MCA (0.1 mg) and quercetin compared with its occurrence in mice given MCA alone. A simultaneous administration of MCA and quercetin significantly increased the in vivo sister chromatid exchanges (SCE) formation of bone marrow cells when compared with its occurrence in the group given MCA alone. These results suggest that quercetin has an enhancing effect on MCA carcinogenesis and that this enhancement may be associated with such a genetic effect as an increased mutation rate in the host.