Two forms of cytochrome P-450 (hepatoma P-450MCI and P-450MCII) were purified from hepatoma 5123D microsomes of tumor-bearing rats treated with 3-methylcholanthrene. Hepatoma P-450MCI had a specific content of 18.4 nmol/mg protein and showed a main protein band with a minimum molecular weight of 56,000 on sodium dodecyl sulfate-polyacrylamide gel. Hepatoma P-450MCII had a specific content of 7.38 nmol/mg protein and a minimum molecular weight of 50,000. The carbon monoxide-reduced difference spectral peak of hepatoma P-450MCI was at 446.5 nm, whereas the peak of hepatoma P-450MCII was at 451 nm. In the reconstituted system, hepatoma P-450MCI catalyzed 3-hydroxylation of benzo[a]pyrene and O-deethylation of 7-ethoxycoumarin, but showed low activities for N-demethylation of benzphetamine and aminopyrine, O-demethylation of p-nitroanisole, and p-hydroxylation of aniline. On the other hand, hepatoma P-450MCII did not catalyze hydroxylation of any of the substrates tested. By Ouchterlony double-diffusion analysis, hepatoma P-450MCI was immunologically indistinguishable from rat liver cytochrome P-450c, but hepatoma P-450MCII was distinct from hepatoma P-450MCI and rat liver cytochrome P-450c. Peptide maps of hepatoma P-450MCI and rat liver cytochrome P-450c after proteolysis with Staphylococcus aureus V8 protease demonstrated the similarity of the two cytochromes P-450.