We studied the oxidative status of red and white blood cells during the development of graft vs host disease (GVHD) as well as the effects of treatment with antioxidants, both in vitro and in vivo. (BALB/c X C57BL/6) F1 mice were conditioned by total body radiation and, 1 day later, transplanted with semi-allogeneic C57BL/6 spleen cells. GVHD was followed by its clinical manifestations. Oxidative stress in red blood cells (RBC), neutrophils, and lymphocytes was assessed by measuring generation of reactive oxygen species and the content of reduced glutathione by flow cytometry after gating of the specific populations. Oxidative stress was noticed 3 weeks after transplantation. It was higher in mice receiving allogeneic spleen cells as compared with mice transplanted with syngeneic cells, suggesting that it was associated with GVHD. The results also demonstrated that treatment with the antioxidants N-acetylcysteine and a derivative of vitamin E (tocopherol succinate, propofol), both in vitro and in vivo, reduced the oxidative stress. The results indicate that various blood cells, including RBC, neutrophils, and lymphocytes, are under oxidative stress and that treatment with antioxidants reduced the stress and, thus, may be useful in ameliorating the severe consequences of GVHD.