Chlorophyll and chlorophyllin, a water-soluble salt of chlorophyll, have been reported to inhibit carcinogen-DNA binding and exert antimutagenic activity for some carcinogenic heterocyclic amines and aflatoxins. In the present experiment, the possible inhibitory effects of chlorophyllin on 2-amino-1-methyl-6-phenylimidazo[4,5-b] pyridine (PhIP) carcinogenicity were investigated. Female F344 rats were administered both PhIP, 0.02% in the diet, and chlorophyllin, 1%, in the diet (group 1), or either PhIP (group 2) or chlorophyllin (group 3) alone for 54 weeks. The incidence of mammary adenocarcinomas induced by PhIP was reduced by chlorophyllin co-administration from 40% (8/20 rats) to 15% (3/20). While the difference was not statistically significant, the multiplicity of adenocarcinomas was significantly (P < 0.05) reduced by chlorophyllin co-administration from 0.50 per animal to 0.15. On the other hand, incidence of colon adenomas was slightly, but not significantly, increased from 10% to 20%. Neither mammary nor colon adenocarcinomas were observed in group 3. Thus, chlorophyllin reduced PhIP mammary carcinogenesis, suggesting that chlorophyllin is an effective chemopreventor when ingested simultaneously with the carcinogen.