Immune complexes are specifically bound to the Fc receptors on the surface of various types of cells capable of presenting antigens. It was therefore determined if human serum albumin (HSA), bound in an immune complex, is presented more efficiently to the HSA-specific T cells than HSA alone. Primed, polyclonal murine T cells were stimulated in vitro with either HSA alone or HSA bound at equivalence to syngeneic, polyclonal anti-HSA antibodies of IgG class. The stoichiometric composition of the insoluble immune complex was AgAb2.91. The present study shows that 10(2)-10(3)-fold lower doses of HSA are sufficient for the same T-cell response in vitro, if HSA is administered to the cultures in the form of the immune complex rather than in the soluble form. The enhancement of T-cell proliferation in the presence of immune complex was blocked with monoclonal antibody (mAb) against Fc receptor. Our results indicate that binding of antigen in the immune complex could play an important role in enhancing an antigen-specific cellular immune response.