Many consider that granular components of platelets should remain intact during storage of platelet concentrates (PCs) in order to participate in hemostasis when infused. The present investigations were done to determine factors affecting the release of an alpha-granular component, platelet factor 4 (PF-4), from platelets stored as PCs. Sedimentation of the pelleted platelet button and stronger flat-bed agitation induced a significant PF-4 release. After 1 day of storage at 22 degrees C, the percent PF-4 release correlated roughly with a pH fall in PCs (r = 0.520, y = 0.014 + 7.204). Rates of pH fall were roughly proportional to the platelet count. In addition, there was a positive relationship between PF-4 release and the platelet count per unit volume (r = 0.615, y = 0.045 + 1.327). The results indicated that one of the critical factors which determines the amount of PF-4 release during storage is concentrated pH. Lowering the platelet count per unit volume in PCs is better for the maintenance of granular components in platelets during storage because of the intact platelet metabolism.