In this electron microscopic study, the melanosomes were always seen as degradative conglomerates within phagocytie vacuoles of the mast cells of skin lesions of dermal melanocytosis and blue nevus. Neither individual premelanosomes nor melanosomes occurring free in the mast cells were observed. The present investigation did not find evidence for melanin biosynthesis in the mast cells, but it suggests phagocytosis and degradation of melanosomes in these cells.