The magnitude of the pH change during platelet concentrate storage at 20-24 degree C in polyvinyl chloride containers is not determined solely by platelet count per cubic millimeter of plasma, since a wide variation in pH was observed with similar platelet concentrations. In modified platelet concentrates having lost through centrifugation 3-15% of total platelets and 61-92% of residual leukocytes, the pH was maintained at substantially higher levels than in the paired control platelet concentrates. Leukocyte levels do not appear to determine the magnitude of the pH fall. Continuous oxygen utilization is needed if the pH is to be maintained near 7.0. However, oxygen tension per se is not the factor which influences the extent of pH change. It has been concluded that a specific platelet subpopulation comprising a small proportion of the total platelets in concentrates and having an enhanced capacity to form lactate may be responsible for a major part of the pH reduction which occurs during storage of many platelet concentrates.