Immune complexes have been prepared using oligovalent hapten-protein conjugates as antigens. Antigens with a valence of less than four exhibited restricted reactivity with antibody in the complement-fixation and precipitin reactions, and formed predominantly soluble immune complexes. The complexes thus formed were characterized with respect to density and antibody: antigen (Ab:Ag) composition. Complexes with a density greater than that of 19S proteins, with an Ab:Ag ratio of 5:2 or more, were found to fix complement more efficiently and to have a relatively short intravascular half-life. Complexes less dense than 19S proteins fixed complement poorly and had a prolonged intravascular existence. In situations where antigenic valence restricted precipitate formation, precipitation could be enhanced by human or rabbit complement, and this appeared to be predominantly an effect of C1q.