Tonsillar lymphocytes from an otherwise healthy nonthrombocytopenic male child were fused with the lymphoblastoid cell line GM 4672. Twenty of 472 (4%) hybridomas had antiplatelet reactivity detected using intact platelets in an enzyme-linked immunosorbent assay. One hybridoma (STO 171) reacted to platelet glycoprotein IIb (integrin alpha IIb) as determined by radioimmunoprecipitation and immunoblotting. Antibody specificity was confirmed using immunodepletion experiments with isotypic antibodies derived from a mutlitransfused Glanzmann's thrombasthenic patient. The antibody reactivity was restricted to platelets and did not react with other integrin alpha-chain proteins expressed on granulocytes or cultured human brain-derived microvascular endothelial cells. These studies indicate that lymphocytes of normal, nonthrombocytopenic individuals have the genetic potential to produce antiplatelet autoantibodies.