Human platelets were preserved by freezing them with 6% DMSO in a mechanical refrigerator maintained at -80° C. The selection of 6% DMSO was arbitrary. After postthaw washing, the 51Cr recovery in vivo was about 45%, the in vitro recovery was about 75%, and the lifespan was about 8.5 days. The mean residual DMSO of 165 mg produced no adverse side effects. The number of platelets in the circulation 2 hr after the infusion of washed freeze-preserved platelets was about half that found with fresh platelets. The prolonged bleeding time induced by aspirin administration to healthy volunteers was corrected within 24 hr of transfusion of a single unit of previously frozen washed platelets; in nine of 12 studies a 50% reduction in the bleeding time occurred within 2 hr of infusion. We do not know though whether washed freeze-preserved platelets will be as effective in correcting the bleeding diathesis in thrombocytopenic patients.