A new isozyme of cytochrome P-450 has been purified to electrophoretic homogeneity from hepatic microsomes of rabbits treated chronically with ethanol. Several criteria indicate that the ethanol-inducible cytochrome, which has a minimal molecular weight of 51,000 and is designated form 3a on the basis of its relative electrophoretic mobility, is distinct from the known isozymes of P-450. As judged spectrally, the new isozyme is high spin in the oxidized state, as is form 4, but differs in that the spin state is unperturbed by nonionic detergents. The absolute spectrum of the ferrous carbonyl complex of form 3a is red shifted as compared to that of forms 2, 3b, 3c, 4, and 6 and exhibits a maximum at 452 nm. The amino acid composition of form 3a is different from that of the other isozymes, and both the NH2- and COOH-terminal sequences are distinct; form 3a has an NH2-terminal alanine and a carboxyl-terminal leucine residue. Peptide mapping by sodium dodecyl sulfate-polyacrylamide gel electrophoresis following treatment with papain, chymotrypsin, or Staphylococcus aureus V8 protease and by high performance liquid chromatography following trypsinolysis indicates that form 3a is a unique gene product. This cytochrome displays the highest activity of all of the rabbit isozymes in the oxidation of ethanol to acetaldehyde and the p-hydroxylation of aniline when reconstituted with NADPH-cytochrome P-450 reductase and phospholipid in the presence of NADPH and oxygen.