The metabolism of 17 alpha-ethinyloestradiol (EE2) to catechol and reactive metabolites by human liver microsomes was investigated. 2-Hydroxyethinyloestradiol (2-OHEE2) was either the sole or principal metabolite. Small amounts of 6-hydroxyethinyloestradiol and 16-hydroxyethinyloestradiol were produced by some of the livers. EE2 (10 microM) underwent substantial (5-20% of incubated drug), though highly variable, NADPH-dependent metabolism to material irreversibly bound to microsomal protein. 2-OHEE2 appeared to be the pro-reactive metabolite. The maximum EE2 2-hydroxylase activity was 0.67 nmol min-1 mg-1 microsomal protein, with a Km value of 8.6 microM. Oestradiol, which is mainly hydroxylated to 2-hydroxyoestradiol, was the most potent inhibitor of hydroxylase activity and exhibited competitive inhibition. Progesterone, which undergoes 2-hydroxylation to a minor extent was also a competitive inhibitor, whereas cholesterol and cortisol did not have any appreciable inhibitory effect. Primaquine was the most potent non-steroidal inhibitor but was non-competitive. Other non-steroidal compounds investigated, e.g. antipyrine, did not show any significant effect on EE2 2-hydroxylation. The results of this study suggest that EE2 2-hydroxylation is metabolised by a form(s) of cytochrome P-450 which has affinity for endogenous steroids.