To evaluate the applicability of a radioactive 125I-anti-IgG test (RIAT) for the detection of small amounts of IgG antibodies on red blood cells (RBC) of patients with autoimmune hemolytic anemia of warm type (AIHA), RBC of 125 patients were studied(AIHA, n=53; Coombs’-negative AIHA, n=6; chronic cold agglutinin disease, n=7; nonimmune anemias, n=59). It was found that the RBC of all cases (33/33) with a positive direct IgG antiglobulin test (DAT-IgG), but also 13 out of 20 patients with a negative DAT-IgG,but detectable complement (C3/C4), and 4 out of 6 cases with Coombs’-negative AIHA gave positive results in the direct RIAT. RBC-associated IgG was higher in the DAT-IgG positive group (n=33; x=8.1%) than in the DAT-IgG negative group (n=26; x3.4%). There was no correlation between hypergammaglobulinemia and RBC-associated IgG. The sensitivity of the indirect RIAT was not remarkably better as compared to the indirect antiglobulin test. The RIAT is valuable in the serology of borderline cases of AIHA.