We previously demonstrated that a novel mutation, characterized by light-colored coats and ruby eyes, which occurred spontaneously in mice in our laboratory, exhibited deletion in the Hps5 gene (ru2^d/Hps5^ru2-d). To clarify the mechanism of this hypopigmentation, the characteristics of the neonatal development of ru2^d/ru2^d melanocytes were investigated in detail with special reference to those of / melanocytes. In ru2^d/ru2^d mice, there were fewer epidermal melanocytes than in / mice, whereas there was no difference in numbers of epidermal melanoblasts in / and ru2^d/ru2^dmice, both in dorsal and ventral skin. Epidermal melanocytes with increased dopa-melanin deposition and dendritogenesis were greatly increased by injecting L-Tyr subcutaneously into newborn ru2^d/ru2^dmice. The eumelanin content in the epidermis and dermis in postnatal ru2^d/ru2^d mice was much lower than in / mice, whereas similar pheomelanin content was observed 5.5 or 7.5 days after birth both in dorsal and ventral skins. Moreover, the eumelanin content in the dorsal and ventral hairs in 5-week-old ru2^d/ru2^d mice was much lower than in / mice, whereas pheomelanin content was two to four times greater than in / mice. These results suggest that the ru2^d allele suppresses the differentiation of melanocytes through the inhibition of eumelanin synthesis, but stimulates pheomelanin synthesis in melanocytes.