The formation of immune complexes was studied by analytical rate-zonal ultracentrifugation using isomolar solutions of rabbit anti-human serum albumin IgG and of the corresponding F(ab')2 fragments. The F(ab')2 fragments retained full ability to react with the antigenic determinants and to form genuine antigen-antibody complexes. Thus, the difference between the IgG and the F(ab')2 systems was supposed solely to reflect the lack of the Fc portion. The complexes formed with F(ab')2 fragments in the zone of low and moderate antigen excess were found to be distinctively mor soluble than those formed with intact IgG. The data indicated that there were two kinds of precipitating immune complexes, namely antibody-rich and antigen-rich complexes. In the antibody-excess zone and the first part of the equivalence zone the immune complexes precipitated due to their antibody richness. Antigen-rich complexes formed in the zone of low antigen excess precipitate only in the presence of antibody-rich insoluble complexes. It is believed that this type of precipitation was due to an Fc-Fc interaction. This new function of the Fc portion of IgG was designated Fc-mediated immune precipitation.