Platelet adherence in bovine endothelial cultures was studied by scanning and transmission electron microscopy. Following incubation with platelet-rich plasma (PRP), platelet adherence to endothelial cell surfaces was rare as compared to similarly-tested fibroblasts which displayed numerous adherent platelets. When endothelial cells were induced to retract from their substrate by exposure to cold or versene and then incubated with PRP, platelets were observed adhering to endothelial cell processes and to an extracellular microfilamentous network, located beneath the cell. Platelets were attached singly, retained their discoidal shape, and showed no evidence of granule release. In contrast, microfilaments and adherent platelets were conspicuously absent in endothelial cultures which were retracted with trypsin or collagenase and incubated with PRP. These preliminary results suggest that the observed interaction between platelets and the subcellular surface of cultured endothelial cells is specific for an extracellular network of microfilaments produced by the cells.