We evaluated a new glucose-free citrate-acetate-NaCl platelet additive solution (PAS 2). In series I, platelet concentrates (PC) were prepared by apheresis and subsequently stored either in plasma (n = 16) or in PAS 2 (n = 15). In series II, PCs were prepared from pools of four buffy coats (BC) and stored in plasma (n = 12) or in PAS 2 (n = 11). By means of ultrastructural morphometry, the volume fractions of alpha-granules, the open canalicular system (OCS) and the fraction of storage granules secreted into the OCS were analyzed during storage for up to 8 days. Additionally, we determined pH, glucose, lactate, pCO2, HCO3-, lactate dehydrogenase and platelet factor 4. Apheresis platelets stored in plasma showed no changes in their contents of alpha-granules and in the fractions of the OCS. In contrast, apheresis platelets stored in PAS 2 displayed a decrease of their relative volume fraction of alpha-granules from 9.1 +/- 1% on day 1 to 3.7 +/- 0.9% on day 5. The fraction of the OCS increased from 7.4 +/- 0.8% on day 1 to 17.1 +/- 1.4% on day 3. On day 8, 93 +/- 9% of all platelets were lysed. Levels of glucose were significantly lower in these preparations and after day 3 glucose consumption decreased to zero. Among PC derived from pooled BC, differences between storage in PAS 2 or plasma were less striking. Only the fraction of alpha-granules secreted into the OCS was significantly greater in BC derived PC stored in PAS 2 on all days. These PCs stored in PAS 2 had a higher plasma carryover (30%) in comparison to apheresis PC stored in PAS 2 (10%). We conclude that plasma is superior to PAS 2 for storage of both apheresis and buffy coat platelets. For preservation of the structural integrity of platelets, the use of PAS 2 requires a minimum of 30% plasma carryover.