Short description of important scientific result: We studied the formation, ultrastructure, and composition of platelet-derived microparticles from isolated human platelets, either quiescent or stimulated with one of the following activators: arachidonic acid, ADP, collagen, thrombin, or calcium ionophore A23187. The structure, dimensions, and intracellular origin of microparticles depend on the cell-activating stimulus. The main structural groups include a vesicle surrounded by one thin membrane or multivesicular structures. Thrombin, unlike other stimuli, induced formation of microparticles not only from the platelet plasma membrane but also from intracellular structures. A fraction of these vesicular particles having an intracellular origin contained organelles, such as mitochondria, glycogen granules, and vacuoles. The size of platelet-derived microparticles depended on the nature of the cell-activating stimulus. T. A. NEVZOROVA, E. R. MORDAKHANOVA, I. A. ANDRIANOVA, L. RAUOVA, R. I. LITVINOV, and J. W. WEISEL