Preliminary data on the biochemical effects of the transfusion of three units of CPD blood in man is presented. Free (ionized) calcium was found to decrease approximately two minutes following initiation of the transfusion of CPD stored blood at a rate of 50 ml/min. Due to mixing problems, arterial and not venous specimens were found necessary to monitor accurately the severity of these decreases. Parathyroid hormone in the peripheral blood was found to increase two to four minutes after the initiation of transfusion. Decreases in blood pressure were observed during transfusion. The blood pressure recovered when the transfusion was stopped and did not decrease any further after restarting the transfusion, even when comparable free calcium levels were achieved. Insulin levels were found not to increase or decrease consistently, and only modest changes in plasma potassium were observed.