In the absence of antigen, all G-subunits which lack the Fc fragment were rapidly eliminated. In the presence of excess antigen, F (ab')₂, Fab and Fd fragments reacted with antigen and remained in the circulation as complexes which were eliminated at the same rate as the antigen. Fab hybrids containing a specific Fd fragment and a nonspecific L chain similarly reacted with antigen and remained in the circulation complexed to antigen. In contrast, L chains and Fab hybrids containing a specific L chain and a nonspecific Fd fragment did not react with antigen in vivo and were rapidly eliminated in both presence and absence of antigen. H chains remained in the circulation of rabbits in the absence of antigen, however, in the presence of antigen, more H chains which formed complexes with antigen remained in the intravascular space and were rapidly eliminated when the immune elimination of the antigen by the host occurred. Nonspecific H chains were rapidly eliminated from the circulaion of guinea pigs, whereas specific H chains remained in the circulation with the antigen.

F (ab')₂ fragments formed complexes with antigen near antibody equivalence and in antibody excess which were rapidly eliminated, however, less effectively than complexes formed near antibody equivalence with intact G. Complexes formed in antibody excess between Fab fragments and H chains remained in the circulation at all concentrations studied and were eliminated at the rate of antigen.

The role of the Fc fragment in the immune elimination of antigen-antibody complexes is discussed.