A plasticizer, di-2-ethylhexyl phthalate, was found to be extracted from polyvinyl chloride plastic blood bags by both human and dog blood stored at 4°C at a rate of 0.25 ± 0.03 mg per 100 ml per day. Plasticizer was found in both the lipid-containing and lipid-free fractions of plasma whereas red cells contained only minor amounts. Seven of 12 lung-tissue samples taken at autopsy from patients who had received transfusions of stored blood contained detectable amounts of plasticizer. One patient, after open-heart surgery, excreted a urinary metabolite of the plasticizer in excess of the amount calculated to have been received in the transfused blood. The additional metabolite possibly came from plasticizer extracted from the polyvinyl chloride tubing of the cardiopulmonary unit. Although the toxicologic implications of these observations are not currently known, it is suggested that plastic formulations that do not contribute any extractable materials to blood or its fractions be developed.