Three established lines of melanoma cells were derived from anatomically distinct metastases occurring in a single patient (DX). The lines, DX-1, DX-2, and DX-3, showed marked phenotypic diversity, as indicated by characteristic differences in growth rate, morphology, pigmentation, and the expression of surface antigens and glycoproteins. DX-1 and DX-3 expressed HLA-DR products, whereas DX-2 lacked HLA-DR expression. DX-1, DX-2, and DX-3 could also be distinguished on the basis of the profile of radiolabeled glycoproteins. Additional quantitative differences in the surface antigenic phenotype of the three cell lines were revealed by serological tests with a battery of monoclonal and conventional antibodies defining melanoma differentiation antigens. In tests for autologous humoral immunity to melanoma cells, sera from patient DX were found to have IgG antibody that reacted with surface antigens of DX-2 cells; no autologous reactivity was seen with DX-1 or DX-3 target cells or with three more recently established melanoma cell lines from patient DX. Absorption analysis indicated that the antigen detected by DX sera on DX-2 cells is a class 1 melanoma antigen, having been detected only on DX-2 cells and in much lower but demonstrable amounts on DX-1 and DX-3 cells. No other cell type, including DX normal fibroblasts, DX B cells, or 45 allogeneic melanoma cell lines expressed the class 1 antigen of DX melanoma. The fact that only one of the melanoma cell lines derived from patient DX was suitable target for the detection of autologous class 1 reactivity has implications for the study of human tumor antigens and may explain why antibody to class 1 antigens has been found so infrequently in past studies of melanoma patients.