Human melanoma cells freshly isolated from 20 patients with primary and 73 patients with metastatic melanomas were analyzed by indirect immunofluorescence staining with monoclonal antibodies (MoAb) to class I (HLA-A, -B, and -C) and class II (HLA-DR and -DQ) antigens and to melanoma associated antigen (MAA). The latter included the GD3-MAA and the high molecular weight MAA. HLA class I antigens were present in 91 and 93% of primary and metastatic tumors, respectively. GD3-MAA was detected in 100% of primary and 80% of metastatic tumors. Whereas the high molecular weight MAA was expressed in 75% of tumors. Sixty % of primary and 50% of metastatic melanomas were stained by anti-HLA-DR MoAb, whereas 38 and 21% of cases, respectively, were stained by anti-HLA-DQ MoAb. Marked phenotypic heterogeneity was evident among primary and metastatic tumors, including different metastases from the same patient. Moreover, in vitro culture of melanoma cells isolated from metastases was associated with an increase from 50 to 75% of tumors stained by anti-HLA-DR MoAb but not of tumors positive for HLA class I antigens and MAA. In vitro incubation with partially purified or recombinant human gamma-interferon enhanced the expression of HLA-DR antigens on all short-term cultured melanoma cells tested but induced and/or augmented the expression of HLA-DQ antigens only in 5 of the 8 cases examined. The average increase in antigenic expression was higher for HLA-DQ than for HLA-DR antigens. Flow cytometric measurement of DNA content of melanoma cells treated with gamma-interferon revealed that the increase of HLA-DR and -DQ expression induced by gamma-interferon was independent from the cell cycle of the tumor cells.