The PFA-100 is a new apparatus used to detect platelet dysfunction in vitro . Anticoagulated blood flows under constant pressure through a capillary, and across an aperture that pierces a membrane coated with collagen and either epinephrine or ADP. Through their ability to adhere and aggregate, platelets occlude the orifice and the closure time is a test of platelet function. Using electron microscopy and immunogold staining, we have analyzed the ultrastructure of platelet aggregates formed within the aperture and that are responsible for the occlusion. Standard electron microscopy showed that the aggregates formed on both collagen-epinephrine and collagen-ADP cartridges presented the same morphological features. The aggregates were exclusively composed of platelets, some of which were degranulated. Degranulation was particularly intense at the periphery of the aggregate where platelets were often totally devoid of secretory organelles. Immunogold staining on ultrathin frozen sections with polyclonal antibodies, allowed us to evaluate the distribution of adhesive proteins such as fibrinogen and von Willebrand factor (vWF) within the aggregate. The latter was found to be abundant in the intercellular spaces between adjoining platelets. Although fibrinogen was also present, its labeling was less intense suggesting that vWF is the major protein implicated in the platelet-platelet interactions in the aggregates formed in the PFA-100 system. This may be because of the high shear rate that occurs across the aperture which suggests that the PFA-100 is particularly sensitive for detecting abnormalities of vWF-platelet interactions.