At least two cytochromes P450 belonging to the CYP3A subfamily may be expressed in adult human liver (1 ), CYP3A4 and CYP3A5. A third enzyme, CYP3A7, is expressed in human fetal liver (2 ). The CYP3A enzymes account for an estimated ~30% of total human cytochrome P450 content in adult liver (3 ), although large inter-individual differences exist in hepatic CYP3A content. CYP3A4 is present in all adult human livers and is inducible by drugs such as rifampin (rifampicin) and dexamethasone (4 –6 ). By contrast, CYP3A5 is expressed in only ~ 10-30% of liver samples (7 ) and does not respond to typical CYP3A inducers (5 , 6 ). Triacetyloleandomycin (8 , 9 ) and gestodene (9 ) are CYP3A-selective chemical inhibitors. Many commonly used drugs are substrates for CYP3A, including erythromycin (10 ), infedipine (11 ) and midazolam (12 ). Immunoinhibition experiments with CYP3A subfamily-specific antibodies have established several microsomal enzyme activities, including nifedipine oxidase (11 , 13 ) and testosterone 6β-hydroxylase (14 , 15 ), as useful catalytic monitors for hepatic CYP3A In a recent study, an inhibitory antipeptide antibody against CYP3A4, which did not cross-react with cDNA-expressed CYP3A5 as judged by Western-blot analysis and did not inhibit cDNA-expressed CYP3A5-catalyzed testosterone 6β-hydroxylation, was found to inhibit virtually all of the testosterone 6β-hydroxylase activity in human liver microsomes (16 ), suggesting that this activity has a high specificity for hepatic CYP3A4.