We have evaluated the effect on the response of citrated platelet-rich plasma to aggregating agents of storage at 4 °C versus room temperature (21 °C) for 2 and 4 h after venipuncture. While there were small decreases in some responses to epinephrine, ADP and collagen attributable to 21 °C storage, only in the case of ristocetin-induced aggregation was a profound difference noted. Platelets stored at 4 °C for 4 h showed no significant change in response to ristocetin. In contrast, those stored at room temperature showed a marked decrease. This change is not likely to be attributable to a change of pH, although pH rose less with storage at 4 °C than at 21 °C. It is recommended that PRP for in vitro testing be stored at