A novel procedure for storing blood at a controlled pH consists in collecting blood in a pH 8.20, Tris-CPD solution and storing it in a special recipient including a gas permeable membrane under a CO(2) atmosphere. The recipient is placed in an atmosphere of variable CO(2)content, so that the initial alkalinity of the preservative is balanced by dissolved CO(2), the proportion of which is diminished when lactate production increases with storage. 2,3-DPG and ATP were studied at three different pH levels of approximately 7.25, 7.45, and 7.65 at 4°C. The best pH for the simultaneous maintenance of 2,3-DPG and ATP was 7.65. Under these conditions, 2,3-DPG is maintained at its initial level and ATP at 55% of its initial level at the 30th day. Lactate production is linear and hemolysis moderate.