Human vitiligo is a common depigmenting disorder that is often associated with polyendocrinopathies. The etiology of vitiligo is unknown although there is indirect evidence of a strong association between antimelanocyte antibodies in animal and human vitiligo. We report direct evidence that vitiligo patients' sera containing antimelanocyte antibodies can lyse cultured human melanocytes by both complement activation and antibody-dependent cellular cytotoxicity (ADCC). Melanocyte cytotoxicity was measured using an ethidium bromide/acridine orange viability assay, after 4 and 16 h incubation with sera from vitiligo patients and from normal controls. Significant melanocyte cytotoxicity was seen with vitiligo patients' sera as an antibody source with both complement-mediated cytotoxicity (p less than 0.01) and ADCC (p less than 0.05) as effector mechanisms. Nine of 11 vitiligo patients' sera produced cytotoxicity by complement-mediated lysis or ADCC. No cytotoxicity was seen using fibroblast targets and vitiligo patients' sera. The lysis of human melanocytes by vitiligo patients' sera by two different effector mechanisms provides direct support for the autoimmune hypothesis of human vitiligo.