The influences of surgical stress on the local graft-versus-host reaction (GvHR) in F1 mice were studied. Skin incision to the F1 mice inflicted to 1 day prior to injection of parental spleen cells produced impairment of popliteal lymph node enlargement, however, this effect was not observed when GvHR was induced on the day 3 and 5 after operation. Spleen cells from hind limb amputated F1 mice exhibited suppressor activity on local GvH reaction when injected into naive syngenic F1 mice together. This suppressor activity was markedly detected 3 and 24 hrs after operation, then gradually reduced and completely disappeared on the day 7. These suppressor activity completely disappeared by the treatment with anti-Thy 1.2. antibody and complement. These results showed that GvH reaction was suppressed by the surgical stress, and this suppression was due to suppressor T lymphocytes in the spleen.