DNAs from human malignant melanoma cells and autologous peripheral blood lymphocytes were evaluated by Southern blot analysis with probes for class I and II HLA genes. DNA of melanoma cells digested with PvuII, EcoRI and BglI and hybridized with a DR beta probe showed a loss of several fragments when compared with DNA from lymphocytes. The same DNAs were not distinguishable when hybridized with a DQ beta probe. Analysis of melanoma and autologous lymphocyte DNAs from the same patient with a class I cDNA, after digestion with several endonucleases, revealed a further loss of fragments in melanoma cells. Comparison of restriction fragment patterns of melanoma and lymphocytes with those of homozygous, serologically-typed cell lines indicated that melanoma cells have lost fragments diagnostic of DR2 and A1 antigens. A densitometric analysis of signals of several oncogenes in comparison with that of DR indicated that a duplication of the remaining DR allele had occurred in melanoma cells.