The authors have generated monoclonal antibodies to an extract of melanoma. When tested on a variety of fixed, embedded sections of malignant tumors, one antibody (HMB-45) reacted with 60 of 62 melanomas and none of 168 nonmelanomas (carcinomas, lymphomas, and sarcomas). The antibody reacts with junctional nevus cells but not intradermal nevi, and recognizes fetal and neonatal melanocytes but not normal adult melanocytes. This antibody thus demonstrates absolute specificity for melanocytic tumors and thus has great utility for the surgical pathologist in distinguishing among poorly differentiated tumors of uncertain origin. It also identifies differences among populations of melanocytes which may be useful in understanding the biology of and interrelationships between these cells.