Two cytochrome P-450 preparations, a constitutive isozyme, form 3b, and a phenobarbital-induced isozyme, form 2, were isolated from rabbit liver microsomes and compared by peptide mapping following digestion with trypsin and by partial sequence analysis. The NH2-terminal sequence of form 3b differed from form 2 in 15 out of 18 amino acids, but both forms have an NH2-terminal methionine residue followed by an acidic residue. Comparisons of many of the tryptic peptides of the two forms by means of high pressure liquid chromatography, as well as amino acid composition and sequence analysis, indicated that peptides from these forms, with one exception, are different. A tridecapeptide, differing only in a methionine (form 3b)/isoleucine (form 2) replacement was isolated from both forms. The amino acid sequence of this peptide is as follows: Met-Pro-Tyr-Thr-Asp-Ala-Val-Ile/Met-His-Glu-Ile-Gln-Arg. Taken together, these data indicate that forms 2 and 3 represent dissimilar gene products. The observation that these two cytochromes share an analogous peptide suggests that this tridecapeptide may contribute structural information necessary for common functional properties.