In 30 patients after kidney transplantation, 6-oxo-PGF1 alpha was examined in unextracted plasma by a specific radioimmunoassay. The material was divided into three groups. Patients with acute transplant rejection showing the highest 6-oxo-PGF1 alpha levels, patients with delayed transplant rejection having significantly lower plasma values and a third group of patients with stable transplant function. In these patients the plasma 6-oxo-PGF1 alpha-values were below the lower limit of detection (70 pg/ml). The enhanced 6-oxo-PGF1 alpha during rejection crisis could be interpreted as a self protection mechanism of vascular tissue which is, however, not sufficient in many cases, to prevent the irreversible rejection of the transplant.