Isologous IgG2a and IgG2b anti-hapten antibodies injected along with trinitrophenylated or fluoresceinated keyhole limpet hemocyanin (KLH) were found to considerably stimulate primary IgG responses to the carrier protein in mice. No such stimulation was observed with IgG1, IgM and IgA anti-hapten antibodies. Depending on the antigen antibody combination, the amplification obtained after a single injection of IgG2-complexed haptenated KLH ranged from 20- to 1000-fold. By comparison, secondary responses were only marginally enhanced (approximately 3-fold) when IgG2-complexed rather than free antigen was used to boost irradiated recipients previously reconstituted with primed spleen cells. The IgG2-mediated enhancement was effective over a wide range of antigen/antibody ratios, did not require the use of high affinity antibodies and occurred whatever the route of injection. The stimulation was specific for the complexed antigen and developed to the same extent whether complexes were formed in vitro or in vivo. A comparable stimulation of the anti-carrier response was obtained with several other haptenated proteins but with formaldehyde-treated diphtheria and tetanus toxoids inhibition rather than stimulation was observed.