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 valuesin vitro, viability and functionin 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 recoveryin vitroof platelets frozen with 4 per cent DMSO was 76 ± 16 per cent; the value was 64 ± 16 per cent for platelets frozen with 5 per cent DMSO. The mean in vivo51Cr recovery of autologous platelets frozen with 4 per cent DMSO was 34 ± 6 per cent, and for platelets frozen with S per cent DMSO it was 33 ± 7 per cent. 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 per cent DMSO, but no improvement in the aspirin‐induced platelet aggreg