The leucocyte adhesion molecules lymphocyte functional antigen-1 (LFA-1; CD11a/CD18) and intercellular adhesion molecule-1 (ICAM-1; CD54) facilitate the cell-to-cell interactions which are required for the initiation of immune responses. The role of this interaction in the response to alloantigen was assessed by comparing the effects of monoclonal antibodies against these molecules to the effects of a soluble form of the ICAM-1 molecule in the mixed lymphocyte response (MLR). In contrast to the well-documented inhibitory effects of anti-ICAM-1 or anti-LFA-1 antibodies on mixed lymphocyte responses, we were unable to block these responses with the soluble form of ICAM-1 (sICAM-1). In contrast, the addition of sICAM-1 to these cultures resulted in a two- to sixfold enhancement in the T-cell proliferative response to alloantigen over the normal response. Unlike previous reports, the biological activity of sICAM-1 was not dependent on generation of a solid-phase form of the molecule. The enhanced proliferative response correlated with an increase in the level of TNF-alpha detected in the MLR supernatants and could be blocked by antibodies to TNF-alpha. sICAM-1 had no effect on proliferation or cytokine production in the absence of alloantigen. These results suggest that antibodies which block ICAM-1/LFA-1 not only block the adhesion which is required to stabilize cell-to-cell contact, but also block the costimulatory signal which is required for T-cell activation.