In order to clarify the time of onset of the differentiation of epidermal melanoblasts and melanocytes in C57BL/ 10J mice, pieces of skin were excised on various days after gestation and subjected to the dopa reaction and to the combined dopa- premelanin reaction. Cells positive to the combined dopa- premelanin reaction ( melanoblast -melanocyte population) were first identified on prenatal day 14 in the dorsal and ventral skin, and increased in number until day 17. The population remained constant (about 140 cells/0.1 mm2 for the dorsal skin and about 65 cells/0.1 mm2 for the ventral skin) until postnatal day 4, and then decreased. However, cells positive to the dopa reaction (melanocyte population) were first identified on prenatal day 16 in the dorsal and ventral skin, and increased until postnatal day 4 (about 95 cells/0.1 mm2 for the dorsal skin and about 25 cells/0.1 mm2 for the ventral skin), then gradually decreased and disappeared by day 30. These results indicate that mouse epidermal melanoblasts begin to differentiate on prenatal day 14, and 2 days later tyrosinase activity is induced within the cells.