Platelet concentrates were prepared in plastic packs of polyvinyl chloride with tri-(2-ethylhexyl) trimellitate as plasticizer. They were stored, with gentle shaking, at room temperature for periods up to 7 days before labelling with isotope and reinfusing. In vivo survival studies, platelet counts, pH and electron microscopy indicated that platelet concentrates prepared in the new plastic were superior to those prepared in the standard pack currently in use. Oxygen was found to diffuse through the new pack more rapidly than through the standard pack. A shelf life of up to 1 week at room temperature seems possible for platelet concentrates prepared in the new plastic.