In an attempt to elucidate the mechanism(s) behind the susceptibility of tissues to the toxic effects of chemicals, NADPH-cytochrome P450 reductase IgG has been used to map the distribution and localization of the cytochrome P450 system in hepatic and extrahepatk human tissues. Employing the Western blot procedure this antibody recognized a single band in human liver mkrosomes which corresponded in mol. wt to the purified reductase. Immunoreactive NADPH-cytochrome P450 reductase staining was detected in all zones of the liver acinus, with maximal staining in hepatocytes adjacent to the terminal hepatic venules (zone 3). Considerable variation in the intensity of reductase staining was observed in different segments of the gastrointestinal tract. Staining was most intense in the enterocytes of the small intestine, with maximal staining at the tips of the villi. Colonic epithelial cells were variably positive while the rectum was negative. Pancreatic ductal cells were positive whereas exocrine cells were negative. In the lung, reductase was detected in bronchiolar and bronchial epithelial cells, Clara cells and alveolar lining cells. In the kidney, the proximal and distal convoluted tubules, the loops of Henle, the collecting ducts in the medulla and the transitional epithelium all stained positively for reductase. The results demonstrate specific cellular localization of NADPH-cytochrome P450 reductase, and hence the cytochrome P450 system, in human tissues. The differential distribution of the reductase within human tissues may lead to a better understanding of the mechanism underlining site-specific carcinogenesis.