The hybridomas No. 165.28T, No. 473.54S, and No. 653.25N derived from the fusion of myeloma cells with splenocytes from mice immunized with cultured human melanoma cells secreted monoclonal antibodies recognizing antigenic determinants maximally expressed on cultured human melanoma cells and freshly explanted melanoma cells. Monoclonal antibody No. 376.74T reacted also with carcinoma cell lines but with a significantly lower titer. Rosette inhibition assay showed that these antigenic determinants were expressed on antigenic structures, which are not associated with beta 2 microglobulin and histocompatibility antigens. Two monoclonal antibodies recognized the same or closely associated antigenic determinants, and the remaining two monoclonal antibodies reacted with distinct antigenic determinants. All four monoclonal antibodies could mediate antibody-dependent cellular cytotoxicity of cultured melanoma cells, but none could mediate complement-dependent cytotoxicity.