Procoagulant activity (PCA) in normal human urine was found to be sedimented by centrifugation at X 100,000g. Therefore, studies were done to identify the structures associated with the procoagulant activity. Transmission electron microscopy of the X 100,000g pellet revealed numerous membrane-bound vesicles as well as fibrous material. Filtration of normal urine through a 0.2-micron filter removed more than 90% of the procoagulant activity. Scanning electron microscopy of the filter surface revealed 0.1 to 1.1 micron particles and fibrous material. By centrifugation at pH 3 and 5 the fibrous material and particles were separated. The procoagulant activity remained with the particles in each case. The fibrous material was shown to be Tamm-Horsfall protein by SDS-PAGE and Western blotting using anti-Tamm-Horsfall protein serum. Purified Tamm-Horsfall protein itself was not procoagulant. Therefore, PCA in normal human urine is associated with particles 0.1 to 1.1 micron in diameter which appear to be lipid membranes in various arrangements.