Mouse skin melanomas were induced in two stage skin carcinogenesis with 7,12-dimethylbenz[a]anthracene as initiator and croton oil as promoter. After approximately 25 weeks of promotion, small black macules of the skin were observed in C57BL, CDF1 and BDF1 mice, and progressively grew with time. Macules less than 2 mm in diameter were localized mostly in the lower portion of the dermis and, histologically, these lesions were consistent with the diagnosis of melanocytoma and were composed of polygonal to round cells loaded with large numbers of melanin granules. The cells were closely packed forming well-demarcated cell-nests with occasional columnar arrangement. In the macules over 2 mm in diameter, the cells were closely packed to form irregularly bordered cell-nests, showing invasive growth into the surrounding tissues. The lesions were diagnosed as malignant melanomas. The incidence of benign and/or malignant melanoma differed among strains: 80% in BDF1, 70% in CDF1, 30% in C57BL/6 and 0% in DBA/2 mice. One example of tumor induced in a CDF1 mouse was transplantable to homologous CDF1 mice.