R24, a mouse monoclonal antibody against GD3 ganglioside, exhibits a wide range of in vitro effector functions. It also has the ability to bind to itself, presumably through homophilic Fab-Fab interactions, which have been proposed to contribute to its high relative avidity for GD3 and to its effector function activity. It is not known which of these characteristics is necessary for the antitumor effects observed in melanoma patients treated with R24. A mouse-human chimeric R24 (chR24) molecule has been constructed in which the GD3-binding site is preserved. Chimeric R24 demonstrates a lower level of binding to GD3 than does mouse R24 suggesting that there may be some differences between the GD3-binding sites of the two mAb or that Fc determinants can contribute to R24 avidity for GD3. The property of homophilic binding is retained by chR24, demonstrating formally that homophilic binding of R24 involves interactions between variable domains. Both R24 and chR24 fix human complement and mediate antibody-dependent cellular cytotoxicity although chR24 was slightly less efficient at the latter. Unlike R24, chR24 was not able to inhibit melanoma cell attachment to plastic surfaces and was not able to activate human T lymphocytes. We hypothesize that chR24 does not bind to GD3 with an avidity high enough to mediate these effector functions.