We have used the techniques of radioimmunoprecipitation (RIP) and Western blot to identify the membrane proteins that bind certain alloantibodies. Anti-PlA1 sera precipitated two bands, corresponding to platelet glycoproteins IIb and III, whether or not calcium was present during the procedure. By Western blot, this antibody bound only glycoprotein III. Anti-PlA1 serum does not precipitate proteins from the platelets of a patient with Glanzmann's thrombasthenia. Two monoclonal antibodies reacting with lymphocyte HLA antigens, as well as sera from highly allosensitized patients, precipitated bands of 38,500 and 13,500 daltons. These bands correspond to the molecular weights of the two subunits of the HLA antigen, as it has been described for other cell types. The patients' sera also precipitated a protein of 72,000 daltons from some platelets. The sera of two patients with quinidine- induced thrombocytopenia precipitated a 138,000-dalton band (glycoprotein Ib-alpha) in the presence of quinidine. The purified IgG antibody from one patient did not require other plasma factors to bind to platelets in the presence of quinidine, while purified antibody from a second patient required plasma factors other than, or in addition to von Willebrand factor. Although several sera from patients with idiopathic thrombocytopenic purpura (ITP) were tested, only one precipitated membrane proteins by the RIP method; this serum identified binding proteins corresponding to glycoproteins IIb and III.