Summary. Microparticles have been prepared from human and animal plateletfree plasma by dilution of the plasma to lower the specific gravity followed by high-speed ultracentrifugation. The protein, cholesterol, and phospholipid content (and cholesterol-phospholipid ratios) of these microparticles have been determined and compared with values obtained with whole platelet homogenates and various isolated platelet subcellular fractions. Immunological investigations on the microparticles and on the density gradient fractions of platelet homogenates suggest that the particles may have their origin either in the surface membrane or within the intracellular membrane structures of the blood platelets. The microparticles have an associated ATP-ase activity which, though lower in specific activity than isolated platelet ‘contractile protein’ (thrombosthenin), has similar characteristics in response to divalent cations, mersalyl and ouabain. Thrombosthenin antisera and antisera to the microparticles both inhibit clot retraction in vitro at high dilution and also inhibit the Mg2+ ATP-ase activity of platelet homogenates and platelet thrombosthenin. When tested against platelet subcellular fractions from sucrose density gradients both these antisera showed precipitating antibodies to the low density membrane fraction. This fraction consists almost entirely of small vesiculated and larger sheet membrane fragments. In ultrastructure, the microparticles resemble the fine granular material present in the interior of thin-walled sacs which may often be seen in electron-micrographs prepared from fresh platelets. These sacs are believed to be the result of either budding of the pseudopodia or herniation of the cytoplasmic contents through the platelet wall.