Platelet concentrates prepared by platelet apheresis were stored in plastic blood bags with different gas permeability properties. Inadequate oxygen supply gave an insufficient adenosine triphosphate (ATP) regeneration and a compensatory increase in glycolysis and lactic acid production, giving a rapidly falling pH. At pH below 6.0 the glycolysis was inhibited, oxygen consumption ceased, and ATP dropped towards depletion. Adequate oxygen supply kept the lactic acid production low with small pH changes only, and allowed a sufficient ATP regeneration. The release of alpha-granular platelet Factor 4 (PF4) was almost total at pH below 6.0, while at intact metabolic function there was a slow release of PF4. Platelet preservation is enhanced by the use of blood bags with adequate gas exchange properties. In our study one polyvinyl chloride plastic (PVC) bag gave poor results, while another PVC bag and a polyolefin bag showed intact metabolism for 5 days and a moderate release of PF4.