SUMMARY Identity for DL-A antigens between donor and recipient shows a favourable influence on graft survival time. Histologically this seems to be attributable to a retardation of both components of the immune response with a preference for the antibody-mediated component (the arteritis). Functionally this is reflected in two different patterns of rejection in the identical and nonidentical situation, respectively. Hyperchloraemia, preceding serum creatinine elevation for 3-7 days, is found to be a distinct sign of rejection in the identical situation. The presence of alloantibodies in the serum of the recipient has only been demonstrated when a difference in DL-A antigens between donor and recipient existed. Results of further experiments testing the influence of several immunosuppressive regimes on the different parameters of rejection might be of great importance for clinical transplantation.