Each of 15 healthy male volunteers was treated with 650 mg of aspirin 24 hours before the autologous transfusion of one unit of freeze-preserved platelets. Freeze-thaw-wash recovery values in vitro, viability and function in vivo, and the bleeding time and platelet aggregation response were measured. The platelets were frozen with 4 or 5 per cent dimethylsulfoxide (DMSO) at an overall rate of 2 to 3 C per minute and were stored at -80 C in a mechanical freezer for up to eight months. They were washed by dilution/centrifugation. The mean recovery in vitro of platelets frozen with 4 per cent DMSO was 76+/-16%; the value was 64+/-16% for platelets frozen with 5% DMSO. The mean in vivo 51Cr recovery of autologous platelets frozen with 4% DMSO was 34+/-6%, and for platelets frozen with 5% DMSO it was 33+/-7%. In both groups the platelet lifespan was normal. There was a significant reduction in bleeding time after the transfusion of a single unit of autologous platelets preserved with either 4 or 5% DMSO, but no improvement in the aspirin-induced platelet aggregation pattern.