The direct Coombs' reaction has been studied in 18 cases of autoimmune hemolytic anemia not associated with elevated cold agglutinin titers. The erythrocyte coating substance was γG-globulin alone in five patients, γG-globulin in combination with complement in nine, and complement alone in four. It has been demonstrated that the heavy chain and both types of light chains are involved in the γG-antiglobulin reaction. Variation in the strength of direct antiglobulin reactions with specific antisera to Bence Jones proteins of type K or type L indicated that the autoantibodies in nine of these patients were a heterogeneous group of γG-globulin molecules. Possible sites of complement fixation to the red cell membrane and to the γ-globulin molecule are discussed in relation to these findings.

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