A continuous cell line of malignant melanoma has been established. The tumor specimen was obtained from a left axillary lymph node of a malignant melanoma originating in the abdominal skin of a 67-year-old woman. The cell line has been designated TM-1. Explant specimens showed positive dopa staining, but after 2 years and 4 months, or 60 generations of cell passage, the dopa staining is now weakly positive. Treatment with DBcAMP that causes marked morphological changes induces melanin production and lowers the proliferative ability of the cells. These changes were reversible. Transplantation of the cells to nude mice resulted in the formation of large tumors consisting of epithelioid cells and spindle cells. Morphologically, the majority of the cell population in vitro was polygonal epithelioid cells, but the cell passage produced a large number of spindle cells. The shape of the TM-1 cells was examined by scanning electron microscopy. Depending upon the culture conditions, either of these cell types could be predominated, indicating that both cells are of the same origin.