IgG antibodies were isolated from erythrocytes of five patients with autoimmune hemolytic anemia (AHA) and one case of hemolytic anemia of unknown etiology. All six persons were heterozygous for human IgG1 allotypes, and thus their serum immunoglobulins were Gm (a+f+). Quantitative analysis of the antibody preparations disclosed that three patients contained only the Gm(a) allotype; the remaining samples showed a predominance of Gm(a) in comparison to the Gm(f) allele. It was concluded that erythrocyte antibodies of patients with AHA are often restricted in respect to their Gm genetic characters with a preference for the Gm(a) allotype. In a second group of experiments, serums from three cases of hemolytic anemia were incubated with Rh+ erythrocytes from a healthy donor. Subsequently, the red cell IgG coat was eluted and analyzed. One sample had only Gm(a+) antibody; a second case showed Gm(a) predominance. The last antibody preparation varied from a disproportionately high Gm(a) concentration to equal allotype concentrations, depending on the antibody/antigen ratio used during the sensitization of the erythrocytes. The latter data suggested that allelic expression was influenced by the immunologic characteristics of the antibody subpopulation tested. No association could be demonstrated between the Ig allotypes and either the Rh specificity of the eluate, or the presence of complement coating on erythrocytes. These findings are similar to a previous report from this laboratory that noted a preference for the Gm(a) allotype in many isoantibodies with Rh specificity. The data suggest an association between certain erythrocyte antibodies and Ig allotypes.