The cytochrome P-450 fraction of microsomes separated on lauric acid AH-Sepharose 4B columns contains about 75% of the microsomal P-450. This was finger-printed by means of two dimensional isoelectric focusing/ SDS-PAGE. Separation of the fraction by highly reproducible, standard procedures on carboxymethyl Sepharose CL6B into four fractions allowed ready isolation and purification of seven forms of P-450, RLM2, 2b, 3, f4, 5, 5a and f5a. Comparison of the four fractions CMI, CMII, CMIII and CMIV revealed qualitative differences in the proteins contained in CMI and CMII of male and female rats. Identification of these proteins revealed RLM2, present in the CMI fraction of adult male rats, is not present in detectable levels in the comparable fraction from females. Similarly, RLM3 and 5 were present in the CMII fraction of male rats but could not be detected in the corresponding fraction of females. Instead, another protein, fRLM4, was found in the females. RLM5a, found in the CMII fraction of males, was also present in females. Examination of the physical properties of these P-450 proteins revealed those isolated in the CMI and CMII fractions to have fairly neutral isoelectric points (7.1–7.6). Based upon the NH2-terminal amino acid sequence, three classes of constitutive forms of P-450 can be recognized. All of the constitutive forms have methionine in position one and leucine in position seven. By comparing sequence homologies, RLM2 and 2b form one sub-class, RLM3, f4 and 5 form a second sub-class, and P-450f and RLM5a form a third sub-class.