Transfusion-associated graft-versus-host disease (TA-GVHD) is a rare but lethal disorder caused when viable donor lymphocytes engraft and proliferate in a susceptible transfusion recipient. Patients with immune deficiency disorders, hematologic malignancies and bone marrow transplants are at risk to TA-GVHD, as are premature newborns and transfusion recipients who are HLA heterozygous for an HLA-haplotype that is shared with an HLA homozygous donor. Irradiation of blood components with 2500 cGy will inactivate donor lymphocytes and prevent TA-GVHD. Platelets and granulocytes are not functionally impaired by this radiation dose, but red cells sustain detectable damage. Red cell units irradiated and stored for 42 days have significantly higher supernatant recovery of chromium-51 labeled cells is sub-optimal. Based on these data, the maximum permissible storage time for irradiated red cells has been reduced to 28 days.