Eighty patients with AR and chronic transplant rejection as well as with a stable transplant function were monitored by a combination of quantification of 111In-oxine labeled platelet uptake in the transplanted kidney (under γ- camera) and platelet survival (PS). In acute phase after transplantation the patients were controlled twice or three times a day until 3-4 weeks (weekly platelet labeling) after the transplantation. The quantification of platelet deposition (index (i): transplant region:contralateral region/cts. per 5 minutes) is demonstrated to be a very useful parameter of early diagnosis of rejection crisis. In AR i reaches values up to 2,00, the PS between 24 and 72 hours. In constant transplant function i below 1,20 are found, indicating that also in patients without any clinical sign of rejection, a platelet deposition and a shortening of PS occurs. In 8 patients with CR (i: 1,201,35; PS: 60-96 hours) and one week continuous i.v. PG2-administration (5ng PG2/kg/min) leads to a decrease in platelet deposition (i: minus 0,10-0,15) and a prolongation of PS. In 4 of these patients also the serum creatinine was decreased between 0,5 and 1,1 mg%. Using thereafter a combination of antiplatelet drugs we try now to retain the platelet deposition at the lower level reached by means of the PGI2application. Similar beneficial data could be obtained by treating acute rejection with PGI2-infusion. The preliminary data of a controlled study and the platelet function behaviour during PGI2 are reported.