Eight different forms of cytochrome P-450 have been isolated from the liver microsomes of male Sprague-Dawley rats and compared with previously characterized forms on the basis of chromatographic behavior, absorption maximum of the carbon monooxide complex, apparent molecular weight upon SDS-disc gel electrophoresis, and substrate specificity. With respect to 2-acetylaminofluorene metabolism in a reconstituted system involving these cytochromes P-450, form c was approximately 16 times as active as form d, which was in turn at least 30 times as active as any of the other forms. The patterns of metabolites obtained with all of the cytochromes P-450 were determined by h.p.l.c. and were all unique. Form d was as active as form c in producing N-hydroxy-2-acetylaminofluorene. This metabolite was observed with only one other form of the cytochrome, i.e., from PB/PCN-E, which gave rise to it at a very low rate.