A hemolysis assay was developed to detect alloantibodies to human immunoglobulin. A total of 1035 serum samples was tested. Anti‐IgM antibodies were found in 8% of 59 normal persons and in 13% of 439 multiparous women, with the highest incidence of 67% in 341 dialysis patients. Although the anti‐IgM antibodies were inhibited by both IgM and IgG, it appeared that they were also inhibited by F(ab')2but not by Fc. Anti‐IgG antibodies were more strongly inhibited by Fc than F(ab')2. These results suggest that anti‐IgM antibodies might be analogous to antiidiotypic antibodies directed to F(ab')2, whereas anti‐IgG antibodies tend to have greater reactiv