Fragment III derived from papain digestion of rabbit gamma globulin inhibits immune hemolysis primarily by its ability to interact with gp C′ and to some extent by interacting with sheep erythrocytes. Peptic digests of normal and antipertussis rabbit gamma globulins do not inhibit immune hemolysis. Peptic digests of rabbit antisheep erythrocyte gamma globulins are capable of agglutinating sheep erythrocytes at a slower rate when compared to that of the intact antibodies. The latter digests will not lyse sheep erythrocytes in the presence of gp C′, because they fix less complement than intact antibody.