Platelets represent small pieces of cytoplasma released from the periphery of megakaryocytes. Therefore, the cells have no nucleus and normal blood platelet fixed in glytaraldehyde in the presence of rutherium red to enhance contrast of the platelet coat (black). The large empty appearing spaces form part of the open canalicular system. The band of microtubles runs peripheral to the membrane-bound granules which appear heterogeneous in size and electron density. The platelet possesses glycogen particles as well as mitochondria and it is able to use either metabolic pathway glycolysis of oridative phosphorylation. The platelets from rat irradiated by γ-ray with 500 rad. appear to be a disorderly array of microtubules in longitudinal and across section. The canalicular system appears excessive and markedly dissended. There is neither granules nor mitochondria in irradiated platelets.