During a period of one year, spleen lymphocytes from cadaver kidney donors were cryopreserved for later investigations. The HLA-D types of 33 donors and the corresponding 39 recipients were determined in six experiments and the reactivities of the lymphocytes of the individual recipients towards those of the donors were assayed in MLC. The MLC experiments included a sufficient number of unrelated responders and stimulators to allow an estimate of the specific MLC reactivity by the stabilized relative response. A strong and significant correlation was found between specifically low reactivity of recipient lymphocytes against donor lymphocytes and graft survival. A correlation was also found between HLA-D compatibility and graft survival. DR typing was performed on the same donor-recipient pairs. In sixteen cases the DR typing of the recipients was initially a technical failure but successful when repeated on cryopreserved lymphocytes. There was a significant correlation between HLA-DR compatibility and graft survival. HLA-A, B matching and pretransplant blood transfusion did not significantly affect graft survival in this material.