The mode of differentiation of epidermal melanocytes was studied by ultrastructural cytochemistry in the skin of newborn mice of strain C57BL/10J. From observations of epidermal melanoblasts and melanocytes, stage I melanosomes, including both unit membranes and inner matrices, appear to be formed from Golgi vacuoles or rough endoplasmic reticulum (RER). Stage I melanosomes were positive to ammoniacal silver-nitrate reaction in the melanoblasts of 1-day-old mice. All stages of melanosomes were similarly positive in the differentiating melanocytes of 2-day-old mice. However, Golgi apparatus, RER, and vesicles were negative. Therefore, it is conceivable that structural proteins, originated from Golgi vacuoles or RER, are developed into specialized proteins and are detected by this reaction in stage I melanosomes. Stage I melanosomes were dopa-negative in the melanoblasts. Stage I and II melanosomes were similarly negative in the differentiating melanocytes. Thus, the melanoblasts are thought to begin production of stage I melanosomes prior to the onset of tyrosinase activity. In the differentiating melanocytes, dopa-melanin depositions were observed in stage III and IV melanosomes, trans Golgi saccules, and small vesicles derived from these saccules, but not in RER. These vesicles were in contact with, or fused to, melanosomes. These findings suggest that tyrosinase may be transferred by Golgi vesicles into stage I and II melanosomes originating from Golgi vacuoles or RER.