The aim of this study is to make it clear whether plasma prostaglandin E2 (PGE2) is a useful parameter or not for prediction and judgement of the graft rejection. We studied the plasma levels of PGE2 in skin graft recipients; a control group of isogeneic grafts with LEW rats, a group of allografts with ACI to LEW and a group of allogeneic graftings with intramuscular administrations of cyclosporin A (CyA) with 20 mg/kg/day from day-2 to day 14. Rejection was judged to be complete when the grafts got dried or appeared graft necrosis. During the first interval after grafting (from 6 hours to the 5th day), significantly high levels of plasma PGE2 with a peak at 12 hours after transplantation (7.8 days prior to 50% skin survival time (ST50), and with 60 times higher value than normal control, can be seen in the allogeneic group. On the other hand, we found no peaks of plasma PGE2 in the immunosuppressed group while using CyA, but a slight elevation was recognized on the 4th day after stopping CyA treatment (7.7 days prior to ST50) with 10 times higher than the normal value. In conclusion, the plasma PGE2 level elevates about 1 week prior to the graft rejection and the serial monitoring may be useful for prediction and judgement of the graft rejection.