An Ig[immunolobulin]G antibody isolated from the serum of a patient with the Benard-Soulier syndrome induced platelet agglutination in the platelet-rich plasma of 50 normal subjects regardless of their ABO, KOa, KOb, HLA or PlA1 types. This antibody was non-reactive with platelets from 3 other Bernard-Soulier syndrome patients. Indirect immunoprecipitation tests using this serum (or purified IgG) and soluble membrane antigens labeled with 125I that were extracted from normal platelets by the nonionic detergent Nonidet P-40 gave a single radioactive peak at 150,000 MW in sodium dodecyl sulfate-polyacryamide gel electrophoresis. The antigenic determinant reacting with this antibody is apparently absent from platelets of Bernard-Soulier syndrome patients and that the deficient molecule is of 150,000 MW. The role of this molecule in subendothelial adhesion and macromolecular-mediated aggregation is discussed.