Immunoprecipitation was performed with platelet-specific alloantisera (anti-Zwa, -Zwb, -Baka and antiserum Luc) and 125I-labelled platelets of a panel of donors typed for these platelet-specific alloantigens. This was done by sensitization of intact, radiolabelled platelets with the antisera, solubilization of the sensitized platelets with Nonidet P-40 and recovery of the immune complexes from the lysate with heat-killed, formalin-fixed Staphylococcus aureus, strain Cowan I. In the case of platelet antibodies of the IgM class, bacteria also preincubated with rabbit-IgG anti-human IgM before treatment of the lysate were applied. Radiolabelled antigens were then eluted from the bacteria by boiling in an SDS-mercaptoethanol-containing buffer. Subsequently, the eluted antigens were analyzed by SDS-polyacrylamide gel electrophoresis. It revealed that both the Zwa and the Baka antigens are located on the glycoprotein-IIb/IIIa complex. The location of the Zwb antigen could not be established in this way. From the intensity of the precipitated glycoprotein bands, it appeared that Zwa is probably a marker of glycoprotein IIIa, Baka or glycoprotein IIb. However, immunoprecipitates of anti-Zwa and anti-Baka sera both carried, to some extent, complexes of glycoproteins IIb and IIIa. Iodinated platelets of a Glanzmann thrombasthenia patient failed to show any material precipitated by these antisera or the antiserum Luc, known to contain antibodies against both glycoproteins.