An immunologic hypothesis is currently proposed as a possible pathogenesis of nonsegmental-type vitiligo. IgG antibodies against melanocyte surface antigens exist in the serum of patients with vitiligo vulgaris. IgG anti-melanocyte antibodies were reported to induce melanocyte damage in vitro by a complement-mediated mechanism and antibody-dependent cellular cytotoxicity. Perilesional melanocytes express major histocompatibility complex class II antigens and a higher intercellular adhesion molecule-1 compared with those in normal skin. The purpose of this study was to determine the role of IgG anti-melanocyte antibodies in the inappropriate expression of major histocompatibility complex class II antigens and intercellular adhesion molecule-1 on melanocytes. IgG anti-melanocyte antibody samples were purified from the individual serum of patients with active vitiligo. After incubation of IgG anti-melanocyte antibodies with cultured melanocytes, the results revealed: (i) IgG anti-melanocyte antibody stimulated HLA-DR expression on melanocytes; (ii) intercellular adhesion molecule-1 expression on melanocytes was significantly induced by IgG anti-melanocyte antibodies; and (iii) IgG anti-melanocyte antibodies induced an increase in interleukin-8 release from melanocytes. The major histocompatibility complex class II molecules expressed in melanocytes can present antigens to CD4 helper cells as antigen-presenting cells and elicit an immune response. Intercellular adhesion molecule-1 is an important adhesion molecule involved in leukocyte and parenchymal cell interaction and thus plays an essential part in immunologic and inflammatory reactions. It is reasonable to speculate that abnormal expressions of HLA-DR and intercellular adhesion molecule-1 on melanocytes by IgG anti-melanocyte antibodies would present vitiligo antigens and allow the antigen-specific immune effector cell attack that results in melanocytotoxicity.