Indium 111 (111In) labelled autologous platelets were used in the diagnostic observation of patients after renal transplantation. 75 patients were collected in 3 groups of 25 cases each. The first group consisted of patients who were examined during the first 4 weeks after transplantation. Group 2 were patients suffering from histologically proved chronic graft rejection. The 3rd group included cases with good and stable transplant function tested at least 9 months after transplantation. By means of a computerized static study the transplants were examined under the gamma-camera concerning a platelet trapping. Thereby a platelet-uptake-index (PUI) was calculated and compared with the platelet half-life-time (t1/2). This diagnostic method turned out to be suitable for early diagnosis of acute graft rejection. Hereby the PUI increased from 1.13 +/- 0.11 to 1.74 +/- 0.17. The further course of rejection episode manifested itself in corresponding changes of PUI. In irreversible rejections the PUI stayed in the high range or increased further. On the other hand reversible graft rejections showed a reduction of PUI, however, in the most cases not to the original basic level. The platelet t1/2 exhibited a strong negative correlation with PUI. Cases suffering from chronic transplant rejection revealed significantly higher platelet trapping by the graft than patients with good and stable transplant function.