The level of expression and interindividual variation in human hepatic microsomal cytochrome P450 (CYP) 2B6 was characterized using a polyclonal antibody (WB-2B6) raised against rat CYP2B1. Immunoblot analysis using cDNA-expressed human CYPs revealed strong cross-reactivity of this antibody with CYP2B6 (limit of detection < 0.05 pmol) and only minor cross-reactivities with human CYP2A6, CYP2D6, and CYP2E1, all of which could be resolved from CYP2B6 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Analysis of human liver microsomes using this antibody revealed immunodetectable CYP2B6 protein in a majority of individual liver samples, with levels up to 74 pmol/mg protein in the CYP2B6-positive samples. Kinetic analysis of cDNA-expressed CYPs identified many of these enzymes as catalysts of 7-ethoxy-4-trifluoromethylcoumarin (7EFC)O-deethylation, but with significantly different apparentK_M values (CYP1A2 < CYP2B6 ∼ CYP1A1 < CYP2C19 < CYP2C9 < CYP2E1 < CYP2A6). By assaying liver microsomal 7EFC O-deethylase activity at a low 7EFC concentration (5 μM) and preincubating human liver microsomes with anti-CYP1A, anti-CYP2C, and anti-CYP2E1 antibodies, we were able to monitor CYP2B6-dependent 7EFC O-deethylase activity in a panel of 17 human liver microsomes and observe a significant correlation (r ² = 0.80) between this activity and CYP2B6 protein content. The ability of CYP2B6 to activate prodrugs and procarcinogens was examined using gene locus mutation assays in CYP2B6-expressing human lymphoblast cells. CYP2B6-expressing cells were found to be more sensitive than control cells to the cytotoxicity and mutagenicity of cyclophosphamide, aflatoxin B₁, and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone. CYP2B6 is thus a widely expressed human liver microsomal CYP that can contribute to a broad range of drug metabolism and procarcinogen activation reactions.