Cells positive to the dopa reaction (melanocytes) as well as to the combined dopa–premelanin reaction (melanoblasts and melanocytes) in the epidermis of C57BL/10JHir-p/p (pink-eyed dilution) mice were fewer and less reactive than in C57BL/10JHir (black, P/P) mice, suggesting that the proliferation and differentiation of p/p melanocytes are inhibited. To confirm the inhibitory effects of p gene on the proliferation and differentiation of epidermal melanocytes, we cultured epidermal cell suspensions of neonatal skins from P/P and p/p in a serum-free medium. The proliferation and differentiation of p/p melanoblasts/melanocytes in primary culture were greatly inhibited as compared to P/P melanoblasts/melanocytes. The morphology of p/p melanoblasts/melanocytes cultured in melanocyte growth medium, though non-pigmented, was similar to P/P melanocytes; namely, dendritic, polygonal, or epithelioid. About 8% of p/p cells cultured in melanocyte growth medium were positive to the dopa reaction, and about 25% were reactive to the combined dopa–premelanin reaction. Eumelanin content in p/p was extremely reduced compared to P/P. The immunocytochemical staining of p/p melanoblasts/melanocytes revealed that they are negative to tyrosinase, but reactive to tyrosinase-related protein (TRP)-1, TRP-2, and c-kit. However, the reactivities in p/p were lower than in P/P. Although the differentiation of p/p melanoblasts was not induced by endothelin (ET)-1, ET-2, and ET-3, the proliferation of p/p melanoblasts was stimulated by them. These results suggest for the first time that p gene exerts its influence on the proliferative activities of mouse epidermal melanoblasts by affecting the regulatory mechanisms dependent on the function of ETs. J. Exp. Zool. 292:351–366, 2002. © 2002 Wiley-Liss, Inc.