Antigen (Ag)-antibody (Ab) aggregates prepared with several different antigens are solubilized by fresh serum at 37 degrees C (complex-release activity of serum or CRA). The rate of solubilization varies in different systems and is strongly influenced by the affinity of Ab for the Ag in the immune precipitate. With a given Ag-Ab precipitate, the maximum amount of complex that can be solubilized by individual sera is independent of the initial concentration of complexes and cannot be increased by prolonged incubation. CRA occurs in the absence of C2 and C4, but not in the absence of C3 and factor B of the properdin pathway. Addition of C2 to C2-deficient serum or C4 to C4-deficient serum enhances CRA. Solubilization does not involve extensive degradation of the complexed antibody, as might be detected by acrylamide gel electrophoresis of released antibody after reduction and alkylation to separate H and L chains. Immune precipitates can also be solubilized by incubation with monovalent fragments (Fab or Fab') of antibodies against determinants of the Ab molecules in the immune precipitate. In contrast, F(ab')2 fragments decrease the solubility of the immune precipitates. In view of these findings, we propose that CRA is mediated by the binding of functionally monovalent C fragments (C3 and C4) onto Ab molecules in the precipitates.