Four forms of cytochrome P-450, tentatively designated PB-1, PB-2, MC-1, and MC-2, were purified from liver microsomes of rats treated with phenobarbital (PB-1 and PB-2) or 3-methylcholanthrene (MC-1 and MC-2). Each purified form showed a single protein-staining band on SDS-polyacrylamide gel electrophoresis giving a minimum molecular weight of 56,000 (MC-1), 53,000 (PB-1), 53,000 (MC-2), or 49,000 (PB-2). PB-1 and MC-1 were the major cytochrome P-450 components inducible by phenobarbital (PB) and 3-methylcholanthrene (MC), respectively. Antibodies prepared against each form of purified cytochrome P-450 did not cross-react with heterologous antigens in Ouchterlony double diffusion tests, confirming the immunological distinctness of the four forms. The CO-compounds of reduced PB-1 and PB-2 had an absorption maximum at 450 nm, whereas those of MC-1 and MC-2 had a maximum at 447 nm. Judging from the oxidized absolute spectra, MC-2 was of high spin type and the others were of low spin type. Amino acid analysis revealed considerable differences among the purified four forms of cytochrome P-450, and the amino acid sequences of their NH2-terminal portions confirmed that the four forms were different proteins. In a reconstituted system containing NADPH and NADPH-cytochrome P-450 reductase, PB-1 and PB-2 oxidized benzphetamine at high rates, but their oxidation of benzo(a)pyrene was much slower than that by MC-1, which catalyzed rapid hydroxylation of benzo(a)pyrene but had low activity with benzphetamine. The quantity of each form of cytochrome P-450 in microsomes was determined by quantitative immunoprecipitation, and selective induction of PB-1 and MC-1 by PB and MC, respectively, was confirmed. Some induction of PB-2 and MC-2 by the corresponding inducers was also noticed. PB group P-450's were not increased by MC treatment, nor were MC group P-450's by PB.