Two forms of cytochrome b5 are present in rat tissues, with a sequence identity of approximately 60% in the cytoplasmically exposed, tryptic fragments (Lederer, F., Ghrir, R., Guiard, B., Cortial, S., and Ito, A. (1983) Eur. J. Biochem. 132, 95-102). It has been suggested that the two isoforms have partially overlapping subcellular distributions, with each form localized to some extent on both endoplasmic reticulum and outer mitochondrial membranes (Ito, A. (1980) J. Biochem. (Tokyo) 87, 73-80). To investigate the degree of specificity of the localization of cytochrome b5 isoforms, we studied their subcellular distributions with antipeptide antibodies, one specific for microsomal cytochrome b5, one specific for outer membrane cytochrome b, and one against a sequence common to the two cytochromes. We first identified outer membrane Cyt b as a tightly bound, Triton X-114-extractable, 23-kDa polypeptide. We then analyzed biochemically characterized rat liver subcellular fractions by Western blotting and found that outer mitochondrial membrane cytochrome b was not present on endoplasmic reticulum membranes. Conversely, microsomal cytochrome b5 was present on outer mitochondrial membranes in extremely low concentration, at a level < 5% of that on endoplasmic reticulum membranes. Thus, the subcellular distribution of microsomal cytochrome b5 is more restricted than previously thought, suggesting that novel posttranslational targeting mechanisms direct it to the endoplasmic reticulum.