We attempted to elucidate the characteristics of melanin in malignant melanomas in oral mucosa by ultrastructural and energy-filtering transmission electron microscopy to examie the morphology and sulfur contents of the melanins in oral lesions of benign melanosis and malignant melanonmas. In lesions of melanosis of the oral mucosa, the essential pathological features are over-production of eumelanin and migration of melanin into keratinocytes. In lesions of malignant melanoma in situ, abnormal melanosomes consisting of many pleomorphic and ellipsoid forms are present. Sulfur distribution is increased, showing an increase in pheomelanin-like melanin. In invasive malignant melanoma the melanosomes assume irregular ellipsoid and spheroid forms, with the presence of compound melanosomes. The contents are eluted by alkali and most contain sulfur, indicating marked increase of pheomelanin-like melanosomes. The morphology and properties of melanosomes are important differential features for melanocyte-derived melanotic lesions including benign tumor, melanosis, and malignant melanoma. These findings habe great significance in the interpretation of early or precancerous lesios.