We have studied the effect of temperature on platelets during storage for tests of platelet function. Aliquots of PRP were stored at constant pH at 37 degrees C, room temperature and 4 degrees C. At intervals up to five hours, samples were taken for estimation of platelet shape, plasma levels of beta-thromboglobulin and 14C-serotonin, and assessment of platelet aggregation in response to a range of concentrations of ADP and collagen. When PRP was stored at 37 degrees C there was a gradual decrease in the aggregation response during the period of storage. At room temperature the decrease was slower but the response to ADP often increased dramatically before decreasing; at this temperature there was pronounced liberation of beta TG while there was none at 37 degrees C. Platelets stored at 37 degrees C were smooth and elliptical when examined by electron microscopy, but those stored at room temperature showed partial loss of discoid shape and formation of some pseudopodia. Storage at 4 degrees C was associated with total loss of discoid shape and formation of many large pseudopodia. Light transmission studies also showed loss of discoid shape at room temperature and 4 degrees C. We conclude that storage at 4 degrees C or at room temperature causes platelet activation. To avoid this PRP should be stored at 37 degrees C prior to tests of platelet function.