The APC:T cell interface can be effectively targeted with immunotherapeutic proteins. We previously described a unique trans signal converter protein, CTLA-4 · Fas ligand (FasL), that has the inherent capacities to tether the T cell inhibitor FasL (CD95 ligand) to the surfaces of B7 (CD80 and CD86)-positive APC (via CTLA-4:B7 interaction), and in so doing, to simultaneously interfere with B7-to-CD28 T cell activation signals. Given the continuing need for agents capable of inducing allograft tolerance without generalized immunosuppression, we have explored in depth the functional activity of CTLA-4 · FasL in human allogeneic MLR. CTLA-4 · FasL inhibits 1° MLR and induces specific hyporesponsiveness in 2° MLR, with both effects only partially reversible with exogenous IL-2. Moreover, the presence of exogenous IL-2 during the 1° MLR does not affect the induction of hyporesponsiveness upon restimulation. Furthermore, CTLA-4 · FasL enables partial activation of allostimulated T cells, reduces the fraction of actively dividing cells, and increases the percentage of dead cells among dividing T cells. Taken together, these findings suggest that CTLA-4 · FasL-mediated inhibition of secondary alloantigenic responses involves both anergy induction and clonal deletion. Thus, CTLA-4 · FasL, a paradigmatic trans signal converter protein, manifests unique functional properties and emerges as a potentially useful immunotherapeutic for modulating alloresponsiveness.