A number of recent reports have described murine monoclonal antibodies that react specifically with the complex formed by human platelet membrane glycoproteins (GP) IIb and IIIa. We show that the IgG L, a previously described human alloantibody isolated from a polytransfused thrombasthenia patient, has similar properties. When used in non- precipitating amounts in crossed immunoelectrophoresis (CIE), 125I-IgG L bound strongly to the IIb-IIIa complex. However, after dissociation of the complex with EDTA, only a weak binding to GP IIb and no binding to GP IIIa was detected. In further studies, increased amounts of IgG L were interacted with 125I-labeled membrane glycoproteins in (a) CIE and (b) classical indirect immunoprecipitation experiments. Although the antibody was able to quantitatively precipitate the IIb-IIIa complex from Triton X-100-soluble extracts of platelet membranes, no precipitation of GP IIb or GP IIIa was observed after divalent cation chelation. Addition of EDTA to immunoprecipitates containing GP IIb- IIIa resulted in dissociation and partial release of both glycoproteins. The interaction of the IgG L with electrophoretically separated GP IIb and GP IIIa was studied using a Western blot procedure in the presence of Ca2+, Mg2+, or EDTA. The presence of divalent cations did not increase the reactivity of the antibody with the individual glycoproteins. Overall, our results show that acquired antibodies to IIb-IIIa, such as the IgG L, may predominantly react with complex-dependent determinants.