Cytochrome b5 is composed of two domains that can be isolated after tryptic cleavage as two polypeptide fragments. One fragment is globular and hydrophilic and contains the heme; the other fragment is rich in hydrophobic amino acids and is essential for recombination of cytochrome b5 with microsomal membranes (Ito, A., and Sato, R. (1968), J. Biol. Chem. 243, 4922; Spatz, L., and Strittmatter, P. (1971), Proc. Nat. Acad. Sci. U.S. 68, 1042). Equilibrium dialysis and sedimentation equilibrium measurements of the binding of deoxycholate, Triton X-100 and dodecyl sulfate show that neither intact cytochrome b5 nor its proteolytic fragments possess high affinity binding sites for any of these amphiphiles. However, each detergent binds to the protein in a highly cooperative manner at concentrations near the critical micelle concentration. Binding measurements using the separated tryptic fragments show that deoxycholate and Triton X-100 (both nondenaturing detergents) bind to the hydrophobic fragment to the same extent as to intact cytochrome b5, and not at all to the polar fragment. Sodium dodecyl sulfate (a denaturing detergent) is bound to both tryptic fragments, but 70% of the detergent is bound to the hydrophobic fragment although it comprises only 30% of the protein mass. Less detailed measurements were made with synthetic and natural phosphatidylcholines, and show that the intact protein is quantitatively incorporated into phosphatidylcholine vesicles, but that no interaction with the polar fragment occurs. These results are interpreted in terms of the hydrophobic domain of cytochrome b5 having a diffuse hydrophobic surface that can act as a nonspecific nucleus for the formation of a micelle with a variety of amphiphilic substances. This domain of the molecule will insert into any available hydrophobic environment, whether it be detergent micelles, synthetic phospholipid vesicles, or the microsomal membrane. The incorporation of cytochrome b5 into the microsomal membrane is only a specialized case of the general property.