A sensitive immunoradiometric assay (IRMA) was used to quantitate the amount of red blood cell-associated IgG (RBC-IgG).The mean RBC-IgG of 91 healthy volunteers was 33± 13 (SD) molecules per one RBC.The correlation between the agglutination strength in antiglobulin test and the number of IgG were very satisfactry.We also tested the amount of RBC-IgG of 27 patients with immune hemolytic anemia of warm type (AIHA).(AIHA, n=17; RBCs with positive antiglobulin test, n=2; RBCs with negative

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... ulin test, n=8; eluates from patients' RBCs).The mean RBC-IgG of 17 patients with antiglobulin test positive AIHA was 998 f 930 (SD) molecules per one RBC.It was ranged from 257 to 3000 molecules per one RBC.Two patients with antiglobulin test negative AIHA (7.4%, 2/27) had 138 and 172 IgG molecules per one RBC, respectively.The indirect antiglobulin tests of RBCs sensitized with 8 eluates showed positive.The RBC-IgG of sensitized RBCs was higher than that of RBCs from healthy volunteers.Using this assay, quantitative studies of RBC-IgG was extremely useful for the detection of small amount of IgG on RBC in case of the antiglobulin test negative AIHA.