We studied the in vivo survival of packed red blood cells (RBC's) which had been warmed using the new technique of admixture with high-temperature saline. Packed RBC's from five normal male subjects were stored in CPDA-1 at 4°C for 14 days. They were then warmed via admixture with an equal amount of saline heated to 70°C. Osmotic fragility, and supernatant hemoglobin and potassium levels of the warmed RBC's were not significantly different from baseline values. Aliquots of the warmed RBC's were labeled with ''Chromium and transfused into autologous donors. Mean radiolabeled RBC survival at 24 hours was 90.2% (S.D. 6.2%), and mean radiolabeled RBC survival time was 25.3 days (S.D. 2.7 days). These results are within the normal range for RBC's stored for 14 days. This study suggests that RBC survival after transfusion is not impaired by admixture blood warming using saline at 70°C.