Investigation of the ultrastructure of platelets of carefully prepared, native platelet rich plasma fixed in gluteraldehyde-osmium has revealed a process not apparent by other technics. Platelet granules undergo transformation into lameller particles with the configuration of lipid micelles. The platelet micelles are released through the platelet cell membrane prior to aggregation. Comparison of the platelet micelles with micelles formed by aqueous dispersion of extracted platelet lipids and brain cephalin revealed similarities of particle size and miceller configuration. The origin of platelet micelles from platelet granules, their release from the cell in early clot development, and their similarity to other lipids active in coagulation are provocative. Platelet micelles may be the active lipid substance contributed by platelets to coagulation, and the lipoprotein complexes of platelet granules the "hidden form" of the platelet lipid in normal circulating cells.