Thirty-three patients with idiopathic thrombocytopenic purpura (ITP) were tested for HLA-A, B and C antigens, platelet antibodies, immunoglobulin levels and ABO blood groups. With one exception, ITP proved not to be significantly associated with the HLA antigens studied; an increased frequency of HLA-A28 was found in chronic ITP patients (50 vs. 18.7% in the control population). An increased incidence of blood group A was found in ITP patients (64 vs. 37.98% in the control population), especially in those with acute ITP (84.7%). A significant reduction of IgG levels was noted in patients with chronic ITP, while below-normal levels of IgA were found in both chronic and acute ITP patients. There was no difference in levels of IgM. Circulating platelet isoantibodies were demonstrated in 67.6% of the ITP patients. No correlation was demonstrated between the presence of platelet antibodies, immunoglobulin levels or HLA antigens.