Abstract The complete amino acid sequence of rat liver microsomal cytochrome P-450d (induced by isosafrole) was deduced by microsequence analysis of the tryptic peptides after separation by reverse-phase HPLC and alignment by comparison to the cDNA sequence reported by K. Kawajiri, O. Gotoh, K. Sogawa, Y. Tagashira, M. Muramatsu, and Y. Fujii-Kuriyama (1984, Proc. Natl. Acad. Sci. USA, 81, 1649–1653) . Results from the two approaches are in complete agreement with the exception of two residues, Lys-30 (Arg in cDNA) and Phe-261 (Ser in cDNA). As previously reported by us ( L. H. Botelho, D. E. Ryan, P.-M. Yuan, R. Kutney, J. E. Shively, and W. Levin (1982, Biochemistry, 21, 1152–1155 ) the NH 2-terminal sequence of the mature protein lacks the NH 2-terminal Met residue. Comparison of the rat cytochrome P-450d sequence with the mouse cytochrome P 3-450 cDNA sequence reported by S. Kimura, F. J. Gonzalez, and D. W. Nebert (1984, Nucl. Acids Res., 12, 2917–2928) reveals a high sequence homology with a total of 32 amino acid differences including six conferring charge changes. Prediction of the secondary structure of cytochrome P-450d yields a maximum of 17 helices, two of which may be poly(Pro)-like helices adjacent to potential membrane-spanning α-helices. Four of the α-helices are sufficiently hydrophobic to traverse the endoplasmic reticulum. The remaining helices are largely amphiphilic. Analysis of the helices in reference to predicted membrane topology suggests that cytochrome P-450d either (1) has one large and one small globular domain separated by a transmembrane domain and anchored by NH 2-terminal and COOH-terminal transmembrane domains, or (2) has one large globular domain anchored at both ends by transmembrane domains.