Total CYP3A content and unbound 1′-hydroxymidazolam formation clearance (CI_f) were measured by Western blot and catalytic analysis of mucosal scrapings from 15 full-length human small intestines. The total amount of CYP3A protein within each intestinal region was calculated as the product of the median specific CYP3A content (pmol/g mucosa) and the median total mucosal homogenate mass. The total amount of CYP3A in human liver was calculated as the product of the median microsomal CYP3A content for 20 different donor livers, a reported value for hepatic microsomal content of 52.5 mg/g liver (Iwatsubo et al., 1997) and a liver weight of 1500 g. Specific hepatic and intestinal microsomal 1′-hydroxymidazolam intrinsic formation clearances (2.86, 5.13, 3.76, and 0.84 ml/min/nmol CYP3A) were calculated from measuredV_max, K_m, and CYP3A content. Total intestinal and hepatic intrinsic formation clearances were calculated from the median microsomal values and the total amount of CYP3A in each respective organ.