Alloantibodies recognize GPIIb/IIIa on platelets from the strain used for immunization. Biotin-labeled platelet membrane proteins from the mouse strain used for immunization were immunoprecipitated with alloantibodies from immunized mice, subjected to SDS–polyacrylamide gel electrophoresis and detected by western blotting with horseradish peroxidase–streptavidin as described in “Methods.” Molecular weight standards are indicated on the left. Shown here are representative results obtained in 2 of 12 studies involving immunization of SPRET (group 3) and PWK (group 4) mice with C57 platelets. Bands identified correlate with those precipitated by monoclonal rat anti-mouse antibody MWReg30 specific for mouse GPIIb/IIIa (left gel). The extra band above the GPIIb band obtained with alloantibody from group 4 has the apparent molecular weight expected for GPIb (170 kDa) but was not further investigated. The lanes were run on the same gel but were noncontiguous.