A new apheresis device using microprocessor control for the collection of a high-purity single-donor platelet concentrate was evaluated, as was the storage of platelets for up to 5 days in a citrate-plasticized polyvinylchloride blood bag. The study was conducted in three phases: collection of platelets for in vitro studies and determination of donor safety; autologous transfusion of platelets in healthy volunteers; and transfusion of platelets in patients requiring platelet transfusion therapy. Donors had mild hypocalcemia and minimal changes in blood counts except for a platelet count reduction from 288 +/- 50 x 10(3) (288 +/- 50 x 10(9)/L) to 217 +/- 43 x 10(3) per microL (217 +/- 43 x 10(9)/L). A mean of 3.36 +/- 1.24 x 10(11) platelets was collected in the mean volume of 214 mL with red cell and white cell contamination in the range of 10(7). Morphology and aggregation were as described previously in stored platelets. Platelet survival data in eight subjects showed a mean recovery of 61 +/- 11 percent and mean survival of 5.03 +/- 1.07 days by a weighted-mean model. Patients transfused with platelets had mean increments of 23,000 immediately and of 8000 at 24 hours; corrected count increments were 6000 at 1 hour and 4000 at 24 hours. The platelets were successful in providing hemostasis to these patients. Clinically useful 5-day-stored platelets are obtained by using this apheresis technology with a functionally closed system and a citrate-plasticized blood bag.