Platelet concentrates were stored at room temperature and the reactivity of platelets to aggregating agents was studied at temperatures between 23 and 39°C. Fresh platelets showed increased reactivity at higher temperatures. When stored platelets were considered, maximal aggregation was not statistically different when compared with fresh platelets in the temperature range of 23–29°C (5μM ADP). In addition, platelets stored either in Terumo or Baxter (PL 146 and PL 732) plastic bags at room temperature, showed an increased ADP-induced aggregation (10μM) when studied at room temperature compared to 37°C. This pattern was confirmed by the difference in released ATP during aggregation. Collagen, although not resulting in detectable aggregation at 25°C, caused a similar release of ATP as that observed at 37°C.Since the observed differences in platelet reactivity seem to be similar using different types of plastic bags, the increased aggregation observed at room temperature is unlikely to be due to a special effect of a storage bag. Since the recovery of ADP aggregability is better at room temperature than at 37°C, ADP-induced aggregation determined at room temperature might be a better index of the clinical results expected when using stored platelets.