Cultures consisting almost entirely of human melanocytes were obtained from epidermal single-cell suspensions by using phorbol 12-myristate 13-acetate (10 ng/ml) in the culture medium. At this concentration, phorbol ester is toxic to human keratinocytes but not to melanocytes. When the seeding density was optimal (0.8-2 x 10(4)/cm2) and the medium contained both phorbol ester and cholera toxin, melanocytes proliferated extensively. Under these conditions, human melanocytes could be passaged serially in vitro and grown in quantity. This cell culture system can thus be used to answer basic questions related to pigment cell biology and may serve as a control for studies of malignant melanocytes.