Recently, it was shown that 10(2)- to 10(3)-fold lower doses of human serum albumin (HSA) are sufficient for the same T-cell response in vitro, if HSA is administered to the cultures bound in the immune complex rather than in the soluble form. In the present study, we analysed the capacity of HSA in the form of immune complexes to elicit specific cellular immune response in vivo. We found that antigen bound in the immune complex with murine, syngeneic polyclonal antibodies elicited the same T-cell response as fivefold higher doses of free antigen. On the other hand, HSA bound in the immune complexes with xenogeneic, rabbit polyclonal antibodies did not enhance anti-HSA cellular immune response. Our results indicate that binding of antigen in the immune complex could play an important role in enhancing an antigen-specific cellular immune response in vivo.