An immunohistochemical study of 40 primary and secondary malignant melanomas in 27 patients, with routinely-used monoclonal antibodies, demonstrated the antigenic pattern exhibited by these tumors, as well as the frequency of aberrant positivities for epithelial cell markers. The following results were found: 95% of melanomas stained positively for S100 protein and vimentin, which is the characteristic immunohistochemical pattern in melanomas. In most cases, the primary tumor and its metastases in a given patient had the same antigenic phenotype. As for epithelial markers, 10% of melanomas stained positively for KL1 and 27.5% for EMA. Despite this relatively high frequency of KL1 and EMA positivities, in practice, simultaneous positivity for S100 protein and vimentin, confronted with the patient's history and with histologic features, firmly establishes the diagnosis of malignant melanoma in virtually all cases.