Indirect platelet radioactive antiglobulin tests using anti-IgG and anti-C3 were performed in 483 individuals. The subjects' diagnoses and clinical states were correlated with sensitizing protein (IgG and/or C3). Sensitivity of the technique was increased by increasing the serum to cell ratio and there was good correlation between direct and indirect antiglobulin tests in 56 patients. Seven percent of normal healthy non-thrombocytopenic untransfused males and 88% of patients in whom autoimmune thrombocytopenia was clinically suspected showed a positive test result. In the indirect antiglobulin test, mean +/- SD fg anti-C3d/platelet was 1.75 +/- 0.28 (N = 58) with serum from normal subjects, 3.91 +/- 1.86 (N = 123) with serum from patients with idiopathic thrombocytopenic purpura and systemic lupus erythematosus, and 2.78 +/- 0.91 (N = 52) with serum from patients with lymphoproliferative disease. The differences were significant with P less than 0.001. Levels of bound anti-IgG were in general accord with those of other reports; there was no significant relationship with serum immunoglobulin levels. When the indirect antiglobulin tests were positive for both IgG and C3, there was a significant correlation between IgG and C3 bound to normal platelets (r = 0.671; P less than 0.01). Although sera from 39% of patients whose thrombocytopenia was not initially suspected to be on an autoimmune basis gave a positive test result, indirect platelet radioactive antiglobulin tests using anti-IgG and anti-C3 may be convenient tools in the evaluation and follow-up of immune thrombocytopenias.