Crossed immunoelectrophoresis of Triton X-100 solubilized proteins from normal and abnormal platelets was performed with rabbit antibodies raised against normal platelets. In Bernard-Soulier platelets protein 13 was not detected, and neither the amphiphilic (probably GP Ib) nor the hydrophilic (glycocalicin) glycocalicin-related proteins were seen when monospecific antiglycocalicin antiserum was used. The most prominent precipitate, 16, and platelet fibrinogen, 24 were not detected in platelets of two patients with type I thrombasthenia, whereas in one patient with type II thrombasthenia fibrinogen was clearly detected, but the amount of protein 16 remained severely reduced. Protein 16 was heavily labeled after lactoperoxidase-catalyzed ¹²⁵I iodination of normal platelets, and was precipitated by IgG-L, an alloantibody from a polytransfused thrombasthenic patient. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) or protein 16 cut out from immunoplates showed two ¹²⁵I-labeled glycoprotein bands, which migrate as GP IIb and GP IIIa. SDS-PAGE of ¹²⁵I-labeled type I thrombasthenic platelets showed no periodic acid-Schiff bands or peaks of radioactivity in the GP IIb and GP IIIa regions, whereas in the GP I region both the periodic acid-Schiff band intensity and the radiolabeling were within the normal range. Autoradiography after crossed immunoelectrophoresis of iodinated thrombasthenic platelets showed that the bulk of radioactivity was bound to protein 17. This glycoprotein, which was also present in normal and Bernard-Soulier platelets, migrates in the GP I region on SDS-PAGE. Thus, the bulk of radioactivity observed in the GP I region after SDS-PAGE is associated with protein 17 and not with glycocalicin.