Serial mixed lymphocyte culture (MLC) was used to monitor the evolution of donor-specific responsiveness over the first 2 years after cadaveric renal transplantation. Lymphocytes obtained from 37 patients at 0, 1, 3, 6, 12, 18 and 24 months following transplantation were assayed in a one-way MLC using donor lymphocytes as stimulator cells. Donor-specific hyporesponsiveness developed in 66% of the patients with functioning grafts. Donor-specific sensitization was noted in 2 patients with functioning grafts and the MLC reactivity remained unchanged compared to the pretransplant value in 8 patients. The patients with donor-specific hyporeactivity tended to remain either free of rejection episodes or experienced early rejection episodes only. Thus, using serial MLC we were able to identify patients who developed hyporeactivity or tolerance to donor antigens. These patients may require less immunosuppression.