Separation of dog femoral and jugular veins from surrounding structures and transient stasis of blood flow for 30 seconds resulted in a lesion characterized by adhesion of neutrophils to the venous endothelium. Neutrophils harvested from this lesion 5 hours later had developed thromboplastic (tissue factor) activity. In immediately removed veins and veins from neutropenic dogs, this lesion did not occur and scrapings of the vein wall did not have thromboplastic activity. In some veins, gross or microscopic thrombosis was seen. It has been reported that neutrophil adhesion to veins also occurs during human surgery. We suggest, therefore, that neutrophils adhering to veins and producing thromboplastin may be a pathogenetic mechanism of deep vein thrombosis in man.