The trapping of 111Indium-oxine labelled autologous platelets by the transplant was recorded by means of a computerised gamma camera and expressed as platelet uptake index (PUI). Eighty-five patients were studied. The PUI increased in acute rejection from 1.13 +/- 0.11 to 1.74 +/- 0.17, and decreased again when rejection was reversed. In chronic rejection PUI was significantly lower than in acute rejection, but still higher than in long term stable grafts. Prostacyclin infusions were given to six cases of acute rejection and 12 of chronic rejection. In the majority of patients an improvement in transplant function and a decrease in platelet trapping could be demonstrated.