The effect of incubation at 37 degrees C, 46 degrees C, 47 degrees C and 48 degrees C for 1 h on the osmotic fragility of blood was determined. Blood taken from volunteers into ACD and CPD was examined after 1, 7, 14 and 21 days of storage. In addition, 22-day-old blood stored in CPD was examined. The osmotic fragility was unchanged after incubation at 37 degrees C and 46 degrees C, slightly increased at 47 degrees C and markedly increased at 48 degrees C. The duration of storage did not have any marked effect. The results suggest that incubation of stored blood at 46 degrees C for 1 h does not cause red cell damage.