Cyclophosphamide (CP) metabolites, rather than the parent compound, show mutagenic activity towards Salmonella typhimurium TA 1535 tester strain when S9 fraction from phenobarbital (PB)-induced rat liver is used as in vitro metabolizing system. On the other hand, inhibition of CP in vitro mutagenicity was observed by adding increasing amounts of beta-carotene (beta-C) to the system. A typical dose-dependent mutagenic response was observed by assaying 24 h urine samples of PB-induced rats injected i.p. with different amounts of CP. Addition of beta-C to urines of CP-treated rats failed to inhibit their mutagenicity. Conversely, a marked decrease in urine mutagenicity was observed when rats were simultaneously treated with the two drugs. These data show that beta-carotene partially inhibits, in vitro and in vivo, CP metabolism via hepatic mixed-function oxidase enzymes to mutagenic species.