The influences of time of storage of platelet-rich plasma (PRP), temperature of storage of PRP, platelet number in PRP, mean platelet volume in whole blood, sex, age, hemoglobin concentration, and different forms of PRP storage on platelet aggregation (PAG) tests, performed with epinephrine, collagen, arachidonate, and ristocetin by a four-channel aggregation profiler (Platelet Aggregation Profiler, Model PAP-4, Bio/Data Corporation, Hatboro, PA 19040, U.S.A.), were evaluated in four groups of subjects (52 men, 22 women, age range 20-85 years, hemoglobin concentration range 8.4-16.8 g/dl). The PRP was stored with or without packed cells, at room temperature or at 4 degrees C, for 0-6 h. The ideal platelet number of PRP for performing the PAG test fell between 150 and 500 x 10(9)/l. If the number was less than 150 x 10(9)/l, the result of PAG should be meaningless. No significant change was noted for up to 6 h when the PRP was stored either at room temperature or at 4 degrees C. Hemoglobin concentration and mean platelet volume did not affect the PAG. However, there was significant but weak correlation (p = 0.0125, r = 0.3696) between age and PAG when using arachidonic acid as the agonist. Men had significantly increased PAG when collagen and ristocetin were used as the agonists. The PRP was stored best at room temperature, without packed cells. In conclusion, to obtain the best result from a PAG test, the PRP should be kept without packed cells at room temperature for no longer than 6 h, and the platelet number should fall between 150 and 500 x 10(9)/l.