R24 is an IgG3 mouse monoclonal antibody which recognizes the ganglioside GD3. Two variants of R24, in which one (V2-R24) or both (V1-R24) light chains were substituted by MOPC-21 light chains, were isolated and characterized. R24 had a 40-fold higher avidity for GD3 than either variant, suggesting that high avidity binding required the presence of two R24 light chains and, thus, divalency. R24 and both variants mediated antibody-dependent cellular cytotoxicity but antibody-dependent cellular cytotoxicity mediated by variants was weak compared to R24. The presence of at least one R24 light chain was required for complement-dependent cytotoxicity; complement-dependent cytotoxicity was mediated by R24 and weakly by V2-R24 but not by V1-R24. R24, but not V1-R24 or V2-R24, inhibited attachment of melanoma cells to plastic and activated T-lymphocytes, suggesting a threshold of avidity required for these biological effects. In a human melanoma xenograft model in nu/nu mice, radiolabeled R24, variants, and isotype-matched control monoclonal antibodies all appeared to localize in tumors (based on tumor:normal tissue ratios), but specific tumor targeting by R24 was generally 3- to 6-fold higher. R24 prevented melanoma outgrowth in nu/nu mice, while V2-R24 induced partial tumor protection. V1-R24 and the negative control monoclonal antibody did not inhibit tumor outgrowth. Antitumor activity of R24 corresponded to avidity and ability to mediate complement-dependent cytotoxicity in vitro.