IgG produced by splenic cells from five patients with chronic immune thrombocytopenic purpura (ITP) and from four control subjects was radiolabelled and incubated with target platelets. Eluates from these platelets were incubated with solubilized proteins from the same platelets immobilized on nitrocellulose paper. After washing, putative antigens were localized by autoradiography. Positive results were obtained in three of the five ITP patients; in two patients three radiolabelled bands of high molecular weight were noted (greater than 200 000) and in the third patient two different bands (molecular weight 140 000 and 105 000) were seen. Control studies were negative. These studies show that patients with chronic ITP produce antibody reactive with antigens associated with human platelet proteins and that the antigenic pattern differs between patients.