It has been demonstrated previously that thrombocytes have a role in renal allograft rejection. The purpose of the present studies was to use fine-needle aspiration biopsy techniques during the early postoperative period to determine whether the specific localization of thrombocytes was correlated with the ultimate outcome of renal transplantation. The thrombocyte morphology and location were evaluated using monoclonal antithrombocyte antibody techniques. The results show that loose aggregates, often in combination with granulocytes, are nearly invariably seen in acute blast-cell-dominated reversible rejections; these aggregates disappear from the graft when the inflammation is overcome. In irreversible rejections in which inflammation continues and increases in intensity, the early loose aggregates are followed and accompanied by aggregation of thrombocytes on the graft vascular endothelial cells. The result suggests that thrombocyte aggregation into the graft is a two-stage phenomenon, and that only thrombocyte-graft endothelial cell aggregates are an adverse prognostic sign indicating an unfavorable course of rejection.