With monoclonal antibodies against cytochrome P-450(5) and P-450(4,5,6), 52 and 54 kDa bands are visualized in microsomes from proximal as well as distal human small intestine. These bands most probably correspond to cytochrome P-450(5) and P-450(4), respectively. This and several other cytochrome P-450 related proteins are present in hepatic microsomes from the same patient. In both hepatic and intestinal microsomes from this patient cytochrome P-450(8) is hardly detectable. In contrast to the small intestine and liver, large intestinal tissue from several other patients does not contain cytochrome P-450(5). Here the 54 kDa isoenzyme, possibly cytochrome P-450(4), is most prominent. Earlier we found a higher content of total cytochrome P-450 in the proximal as compared to the distal small intestine. A similar distribution is now found with regard to aldrin epoxidase activity.