The morphologic differences of two melanoma cell cultures from a primary melanoma and a metastasis are described. Though both cultures looked very similar at the beginning, they developed into totally different forms. Long, thin spindle-shaped cells containing an optically dense plasma grew in the culture from the primary melanoma during its whole life span and the epithelioid cells of metastasis culture separated itself into different subcultures. The cells of both subcultures detached from the bottom of the culture flasks after 15 days of growth and began to spread again 7 days later with two very different forms. This transformation was observed three more times at monthly intervals. The possible relationship between these morphological alterations and the ability of the melanoma cells to produce metastases as well as the usefulness of melanoma cell cultures for immunodiagnostic purposes are discussed.