Regional blood centers frequently need to hold units of whole blood at 20 to 24 degrees C for several hours after phlebotomy so that sufficient platelet concentrates can be prepared to meet the increasing need. We have evaluated the in vivo viability and in vitro properties of platelets that were prepared from whole blood drawn into citrate-phosphate-dextrose-adenine (CPDA-1) either immediately after phlebotomy or after an 8-hour hold at 20 to 24 degrees C. Platelet concentrates were stored for 5 days at 20 to 24 degrees C in polyolefin containers (PL 732, Fenwal) with end-over-end tumbler agitation. The autologous in vivo recovery (mean +/- SD) and one-half disappearance of 51Cr-labeled platelets prepared immediately after phlebotomy were 44.4 +/- 9.4 percent and 4.0 +/- 0.5 days, respectively. Platelets prepared after the delay of 8 hours showed a recovery of 44.5 +/- 8.4 percent and a one-half disappearance of 4.1 +/- 0.4 days. After 5 days of storage, platelet concentrates showed a mean pH of 7.21 +/- 0.20 when prepared immediately after phlebotomy, and of 7.22 +/- 0.15 when prepared after an 8-hour delay. Mean morphology scores were 280 +/- 33 and 302 +/- 27 for platelets from units prepared immediately after phlebotomy or after a holding period of 8 hours, respectively. Platelets underwent synergistic aggregation after 5 days of storage, independent of the length of time that the units of whole blood were held prior to centrifugation. These studies indicate that platelet concentrates prepared from units of whole blood held initially for 8 hours can be stored for 5 days at 20 to 24 degrees C and survive satisfactorily in vivo and retain in vitro characteristics.