Proliferative and cytolytic lymphocyte responses and the influence of exogenous interleukin 2 (IL2) on cell-mediated lympholysis (CML) reactivity were evaluated in 12 allograft recipients. Responses were induced by mitogenic lectins or by donor and third-party cells. Patients were tested immediately before transplantation (Tx) and one and three months after grafting. Prophylactic immunosuppression consisted of Cyclosporin A (CyA) and low-dose prednisone (P). Analysis of post transplant cells revealed a reduced overall proliferative T cell responsiveness induced by both alloantigens and mitogenic lectins. No evidence for donor-specific reduction of MLC responses was seen. Overall CML reactivity of post-Tx lymphocytes was also impaired. This was accompanied by donor-specific CML non-reactivity in six of seven patients with quiescent grafts. In these patients, the cytolytic potential against donor cells could be restored when maximal T cell help via exogenous IL2 was provided.