131 patients (124 with idiopathic thrombocytopenic purpura) have been investigated for platelet autoantibodies with an antiglobulin consumption assay (ACA) and a direct Coombs test (DCT) using 125I-antiglobulin. The two assays have been simultaneously performed on 165 platelet-suspensions. The results of ACA (expressed as normal or abnormal amounts of platelet bound IgG or PBlgG) and those of DCT (expressed as positive or negative) were concordant in 128 out of 165 cases (77.6 per cent). In 37 of 165 cases (22.4 per cent) the DCT was negative while PBgG was found to be slightly elevated with ACA. There was a significant correlation (r = 0.44) between PBlgG levels and radioactivity index calculated in DCT. The relationship between these data was graphically expressed as a curve constructed on semilog paper meaning that PBlgG levels corresponding to high radioactivity index were more uncertain than those corresponding to lower index. The quantitative expression of DCT would be improved by refering the results to a standard curve and not only to normal platelets. In the case, results of DCT would be probably equivalent to those of ACA.