The irradiation of blood components with X or gamma rays is necessary to prevent the graft-versus-host disease, but it also provokes untoward effects. In particular, red cells are damaged and have a decreased in vivo recovery, an increased in vitro haemolysis, and a leakage of potassium in the supernatant. The results of the clinical studies show that the loss of viability progressively increases with the storage after irradiation. On the other hand, the storage before irradiation is inconsequential. The mechanism through which irradiation causes the loss of viability is unknown, but a critical examination of the literature and our results indicate that the erythrocyte deformability is the only parameter related to viability to show sufficiently precocious and important changes. We also tried to identify the mechanism by which irradiation influences deformability and examined, in particular, the changes in the mean cell volume (MCV) and vesiculation. However, the temporal behaviour of both suggests no causal relationship.