Immunoprecipitation of srGPIIb and srGPIIIa by human antibodies specific for the alloantigens Pl^A1 (HPA-1a), Pen^a (HPA-4b), and Bak^a (HPA-3a). As predicted from the known amino acid sequences of srGPIIb and srGPIIIa, anti-Pl^A1 and anti-Pen^a recognized srGPIIIa and anti-Bak^a recognized srGPIIb. No bands were obtained with normal serum (not shown).