Cloudman S91 mouse melanoma cells express both external (plasma membrane) and internal binding sites for MSH. Using '"I-beta melanotropin (P-MSH) as a probe, we report here an extensive series of studies on the biological relevance of these internal sites. Cells were swollen in a hypotonic buffer and lysed, and a particulate fraction was prepared by high-speed centrifugation. This fraction was incubated with '"I-P-MSH with or without excess nonradioactive P-MSH in the cold for 2 hours. The material was then layered onto a step-wise sucrose gradient (8-80%) and centrifuged (1 56,OOOg, 60 min); fractions were collected and counted in a gamma counter or assayed for various enzymatic activities. The following points were established: 1 ) Specific binding sites for MSH were observed sedimenting at an average density of 50% sucrose in amelanotic cells and at higher densities in melanotic cells. 2) These sites were similar in density to those observed when intdct cells were labeled externally with 'L51-P-MSH and then warmed to promote internalization of the hormonc. 3 ) Most of the internal binding sites were not as dense as fully melanired melanosomes. 4) In control experiments, the MSH binding sites were not found in cultured hepatoma cells. 5) Variant melanoma celis, which differed from the wild-type in their responses to MSH, had reduced expression of internal binding sites even though their ability to bind MSH to the vuter cell surface appeared normal. (MSH-induced responses included changes in tyrosinase, dopa oxidase, and dopachrome conversion factor activities, melanization, proliferation, and morphology.) 6) Isobutylmethylxanthine, which enhanced cellular responsiveness to MSH, also enhanced expression of internal binding sites. The results indicate that expression of internal binding sites for MSH is an important criterion for cellular responsiveness to the hormone