The effects of psoralen derivatives on cytochrome P-450 have been studied in human liver microsomes. CO-binding cytochrome P-450 was decreased by 33% after 10 min of incubation with 1.5 mM EDTA, an NADPH-regenerating system and 20 microM methoxsalen (8-methoxypsoralen). No destruction of cytochrome P-450 was observed when either NADPH or methoxsalen was omitted. A similar (27%) decrease in CO-binding required a 100-times higher concentration of allylisopropylacetamide (2 mM). The activities of 7-ethoxycoumarin deethylase and benzo(a)pyrene hydroxylase were decreased by about 50% in the presence of 12.5 microM methoxsalen. At this low concentration, neither cimetidine nor SKF 525-A or piperonyl butoxide had any significant inhibitory effect. Monooxygenase activities were also decreased in the presence of 12.5 microM bergapten (5-methoxypsoralen) or 12.5 microM psoralen, but not with 12.5 microM trioxsalen (trimethylpsoralen). CO-binding cytochrome P-450 was not decreased after 10 min of incubation with 1.5 mM EDTA, an NADPH-regenerating system and 20 microM trioxsalen. We conclude that methoxsalen is an extremely potent suicide inhibitor of cytochrome P-450 in human liver microsomes. Bergapten and psoralen are also inhibitory whereas trioxsalen has little effects. In the latter derivative, a methyl group is attached on the furan ring and may hinder its metabolic activation and the inactivation of cytochrome P-450.