Serum pancreatic secretory trypsin inhibitor (PSTI)* was measured in the course of canine segmental pancreas allotransplantation without immunosuppression. Serum PSTI concentrations showed two distinct elevations: the first elevation was on the first day and the onset of the second elevation was the sixth day after operation. The first postoperative elevation of the serum PSTI level is thought to be related to the operative procedures, because the first elevation was observed after both autotransplantation and allotransplantation, and biopsies of the autograft and allograft at the first day after the operations showed nonspecific neutrophilic infiltration and no perivascular lymphoid infiltrates. The second postoperative elevation of the serum PSTI level is thought to reflect a rejection process because this elevation was not seen after autotransplantation, and biopsies of allografts at the sixth day after the operation showed typical perivascular lymphoid infiltrates and cellular rejection of the exocrine tissue. In addition, the onset of the second elevation of serum PSTI level preceded by about three days the onset of the elevation of blood sugar at the ninth day after the operation. The results suggest that serum PSTI can be used as a marker for diagnosis of early pancreatic allograft rejection.