Expression of heterophile antigens was studied on 6 human melanoma cell lines. Paul-Bunnell and Hanganutziu-Deicher (H-D) antigens, but not Forssman antigen, were demonstrated on these cell lines. H-D antigen was also demonstrated on melanoma cells isolated from primary and metastatic lesions. Evidence was also presented that HLA class I but not class II (DR and DQ) molecules are expressed on these cell lines. H-D antibodies of IgG class were detected in 10 of 23 (42%) melanoma patients suggesting the possibility that H-D antigen might have been expressed in an immunogenic form in the patients.