Transfusion-asseciated gruft-versus-host disease(TA-GVHD) is usually a fatal outcome of blood transfusion therapy caused rnby viable leucocytes contained in the donor blood. Most cases of TA-GVHD occur when less than 4-d-old blood is transfused. We rntherefore examined the molecular changes that occur during storage that may account for the paucity of TA-GVHD following infusion rnof older blood. Leucocyte number and viability were essentially unchanged from freshly obtained blood, but the expression of cell-surrnface lymphocyte activation antigens(CD3,CD4,CD28,CD2,CD45) decreased rapidly within the first 24 h and continued to fall to less rnthan 20% of original levels by d 9 of storage at 4℃. The decrease in CD antigen expression directly correlated with a decreasing ability rnto induce activation of the T-tymphocyte cellular signal transduction pathway. As a result, sells became less responsive in a mixed lymrnphocyte culture(MLC) by d 3, with abrogation of the MLC responsiveness by d 5. Donor leucocytes stored for 4 d or less at 4℃ were rnable to partially re-express. CD antiger and reconstitute, their signalling pathway when plaued at 37℃, where as those stored for more rnthan 4 d were not. These irreversible changes result from a permanent down regulation of donor cell protein synthesis. These findings rnprovide a mechanism to explain the paucity of TA-GVHD following transfusion of blood that is more than 4 d-old. Further study may rnshow that aged blood provides additional assurances for the prevention of TA-GVHD; hoever, use of aged blood should not replace currnrent protocols using irradiation.rn