R24, a mouse IgG3 mAb against GD3 ganglioside, was shown to bind to itself in a homophilic manner. This was demonstrated by augmented binding of 125I-labeled R24 to the cell surface of GD3+ cells by unlabeled R24 and by direct binding of biotinylated R24 to R24 adsorbed on solid phase. Although homophilic binding was evident when R24 was bound to solid phase, R24-R24 aggregates could not be detected in solution under otherwise identical conditions. R24 bound to four other mAb (two IgG3, one IgG2a, one IgM) directed against GD3 but did not bind to a panel of 21 other mAb including other IgG3 mAb and mAb directed against non-GD3 ganglioside. Evidence implicating the GD3-binding site of R24 in homophilic binding included the following observations: 1) F(ab')2 fragments of R24 could bind to R24, 2) an antiidiotypic mAb against the GD3-binding site of R24 inhibited R24 homophilic binding, 3) an IgM anti-GD3 mAb also demonstrated homophilic binding to R24, and 4) homophilic binding was a function of immunoreactivity and avidity for GD3. R24 variants with 40-fold lower avidity for GD3 demonstrated a similar decrease in homophilic binding. Inasmuch as R24 bound to R24 F(ab')2 fragments and specifically to anti-GD3 mAb, it appeared that the target for homophilic binding was an epitope within the V region of anti-GD3 mAb. It is likely that homophilic interactions result in increased affinity of R24 for GD3 through increased effective valency of antibody-Ag complexes.