Using a combination of gel permeation chromatography and freeze fracture electron microscopy it was found that the ability of sulfatides to promote contact activation in human plasma is associated with the ability of sulfatides to form bilayer structures. When sulfatides were dispersed in buffer at 70 degrees C by vortexing, large liposomes were found ranging in size between 2-5 micron diameter. Contact activation promoting activity was associated with these particles. Sonication resulted in the formation of small vesicles and stacked vesicles of 400-2000 A diameter and biological activity was associated with these vesicles. Homogeneous preparations of small unilamellar vesicles (500 A diameter) can be obtained by chromatography over Sepharose 2B.