Red cells were prepared using a new anticoagulant with half the normal amount of citrate and a new additive solution (RAS2, Erythrosol ( R)) previously shown to give improved storage conditions, and stored in a highly gas permeable plastic container (PL 2209). Mixing daily and weekly resulted in lower PCO 2 , higher PO 2 and more rapid oxygen saturation of the haemoglobin than storage unmixed in an upright position. Storage horizontally in a lying position unmixed resulted in similar blood gas values as with mixing.The haemolysis was lowest in units mixed once weekly: 0.21 +/- 0.09% after 28 days and 0.26 +/- 0.06% after 56 days. The morphology was better maintained in mixed than in unmixed units. Horizontal storage and mixing once per week seem to be optimal as judged from thesein vitro studies.