There are no large-scale data available describing the increments obtained with platelet concentrates stored for varying durations. Platelet concentrates prepared by standard techniques were stored at 22 degrees C with horizontal agitation in PL-732 bags and administered to clinically stable, nonalloimmunized recipients known to respond well to random donor platelet transfusions. The platelet concentrates were stored in a mean volume of 65.0 mL (range 54–80 mL) with an average yield of .72 X 10(11) platelets per unit of platelet concentrates (N = 100 consecutive units). There was no significant deterioration of pH during storage. Mean corrected count increments ranged between 16,600 (N = 146 transfusions) after 1 day of storage to 13,300 (N = 34 transfusions) after 5 days of storage. Although these differences were statistically significant (P less than .003, analysis of variance), the overall deterioration in increments was quite modest. Platelet concentrates can be stored under standard conditions for 1 to 5 days with acceptable clinical results.