HMB-45 (melanocytic cell-specific monoclonal antibody) immunoreactivity has been detected by the immunoperoxidase technique in the cytoplasm of 2 of 100 breast carcinomas. In addition, normal breast lobules and ducts were occasionally found to have positive results in 6 of 100 cases. These findings indicate the need to exercise caution in the interpretation of HMB-45 immunoreactivity for immunohistochemical characterization of neoplasms.