We have studied coumarin 7-hydroxylase (COH) and testosterone 15 alpha-hydroxylase (15 alpha OH) activities in human liver microsomes and compared them with corresponding activities catalysed by members of the P450IIA sub-family in DBA/2N mouse liver microsomes. Human liver contained low levels of 15 alpha OH (about 5-30 pmol/min/mg protein) when compared with control mouse liver microsomes (about 200 pmol/min/mg protein). The anti-P450Coh antibody efficiently inhibited mouse liver 15 alpha OH, also 7 alpha OH (which is a member of the P450IIA sub-family), but it did not inhibit human 15 alpha OH or other testosterone hydroxylases. In mouse liver microsomes, metyrapone preferentially inhibited 15 alpha OH, but in human liver microsomes it inhibited all testosterone hydroxylations measured, including 15 alpha OH (IC50 = 2.0-5.0 microM). Metyrapone clearly inhibited COH in mouse liver microsomes, but interestingly it had no effect on COH activity in human liver microsomes, although these two isozymes have earlier been shown to be immunologically similar. On the basis of available evidence human and mouse P450Coh isozymes seem to be orthologous enzymes whereas the present results indicate that the human 15 alpha OH is different from the mouse P45015 alpha.