Antigen-antibody complexes formed at equivalence and injected into adult mice sensitize the mice so that a markedly enhanced antibody response is obtained following an injection of soluble antigen 3 days after the complex injection. The serologic specificity of the enhanced response is determined by the antigen injected on day 3; thus, rabbit anti-dog serum albumin (DSA) in complex with DSA sensitizes the mouse so that when human serum albumin (HSA) is injected on day 3 an enhanced antibody response to determinants unique to HSA is obtained. The antigen injected on day 3 must be related to the antigen in complex in order to produce an enhanced response; tetanus toxoid given on day 3 following injection of DSA-anti-DSA did not stimulate an anti-tetanus response. A mechanism is proposed in which the day 3 antigen-specific bursal-equivalent (B) cells produce an enhanced antibody response as a result of interaction with an increased number of carrier-specific thymus-derived (T) cells which have been stimulated to proliferate by the antigen-antibody complex; in this scheme, the albumin determinants which stimulate T cells to proliferate differ from those which stimulate antibody formation, i.e., B cells.