Evidence is presented that the F(ab')2 fragment of rabbit anti-erythropoietin forms immune complexes with erythropoietin. The in vivo biologic activity of the hormone remains when complexed to the antibody fragment, whereas the hormone has no in vivo biologic activity when complexed with the intact antibody. It is concluded that the effector functions of the Fc fraction of the antibody molecule play an important role in the ability of the intact antibody to neutralize the in vivo biologic activity of the hormone. It is proposed that this role may relate to differences in the rate of catabolism of the different antibody-hormone complexes. It is also concluded that the biologically active sites of the hormone and its antigenic determinants do not correspond.