Platelet plugs were formed in an extracorporeal unit from flowing venous blood and studied by electron microscopy. The unit consisted of a stainless steel needle threaded into a section of silicone rubber tubing that was constricted to form a slit-like stenosis equivalent in cross-sectional area to an arteriole 100 mu in diameter. Blood was allowed to flow at a steady pressure from an antecubital vein through a collection line and the attached unit until bleeding was stopped by the formation of a platelet plug at the stenosis. Electron microscopy of the plugs showed closely packed aggregated platelets. No fibrin was detected. The formation of a stable plug in the absence of fibrin was considered a measure of the capacity of platelets in hemostasis and thrombosis to aggregate and resist the force of the blood current.