During their 7-9 day lifespan in the circulation platelets are mainly responsible for maintaining the integrity of the vasculature. In thrombocytopenic states, there is an increase in vascular permeability and fragility, presumably due to absence of this platelet function. In sharp contrast, biochemical or physical injury in the coronary, carotid or peripheral arteries induces platelet activation and platelet recruitment, which can culminate in thrombotic vascular occlusion. Since there is one death every 33 s from vascular occlusion in the United States, this situation constitutes a major public health issue. In the course of studying interactions between cells of the vascular wall and those in the circulation, we observed that platelets in close proximity to endothelial cells do not respond to agonists in vitro. Experiments initiated in the late 1980's cumulatively indicated that endothelial cell CD39--an ecto-ADPase--was mainly responsible for this phenomenon. CD39 rapidly and preferentially metabolizes ADP released from activated platelets. ADP is the final common pathway for platelet recruitment and thrombus formation, and platelet aggregation and recruitment are abolished by CD39. Our current hypothesis is that CD39 will be a novel antithrombotic agent for treating high risk patients who have activated platelets in their circulation--the identifying characteristic of coronary artery occlusion and thrombotic stroke. A recombinant, soluble form of human CD39 has been generated. This is solCD39, a glycosylated protein of 66 kDa whose enzymatic and biological properties are identical to the full-length form of the enzyme. In our in vitro experiments, solCD39 blocks ADP-induced human platelet aggregation, and inhibits collagen- and thrombin receptor agonist peptide-induced platelet reactivity. We studied solCD39 in vitro in a murine model of stroke, which was shown to be driven by excessive platelet recruitment. In studies with CD39 wild-type (CD39+/+) mice solCD39 completely abolished ADP-induced platelet aggregation, and strongly inhibited collagen- and arachidonate-induced platelet reactivity ex vivo. When solCD39 was administered prior to transient intraluminal middle cerebral artery occlusion, it reduced ipsilateral fibrin deposition, decreased (111)In-platelet deposition, and increased post-ischemic blood flow 2-fold at 24 hours. These results were superior to those we obtained with aspirin pre-treatment. CD39 null (CD39-/-) mice, which we generated by deletion of exons 4-6 (apyrase conserved regions 2-4), have a normal phenotype, normal hematologic profiles and bleeding times, but exhibit a decrease in post-ischemic perfusion and an increase in cerebral infarct volume when compared to genotypic CD39+/+ controls in our stroke model. "Reconstitution" of CD39 null mice with solCD39 reversed these pathologic changes. Thus, the CD39-/- mice were actually rescued from cerebral injury by solCD39, thereby fulfilling Koch's postulates. These experiments have led us to hypothesize that solCD39 has potential as a novel therapeutic agent for thrombotic stroke. In this review, we summarize our recent research results with CD39 and solCD39, and discuss our viewpoints on its present and future possibilities as a novel treatment for thrombosis.