The excessive production of reactive oxygen species (ROS) associated with inflammation leads to oxidative stress, which is involved with the high mortality from several diseases such as endotoxic shock and can be controlled to a certain degree by antioxidants. The immune cells use ROS in order to support their functions and, therefore, need adequate levels of antioxidant defenses in order to avoid the harmful effect of an excessive ROS production. In the present work, the effect of the administration of the antioxidant N-acetylcysteine (NAC) on the redox state of peritoneal macrophages and lymphocytes from mice with lethal endotoxic shock (100 mg/kg i.p. of lipopolysaccharide, LPS), was studied. In both types of immune cells at 0, 2, 4, 12 and 24 h after LPS injection, an increase of ROS, of the proinflammatory cytokine tumor necrosis factor alpha (TNFalpha), the lipid peroxidation (malonaldehyde levels, MDA), inducible nitric oxide synthase (iNOS) expression and the oxidized/reduced glutathione (GSSG/GSH) ratio, as well as a decrease of enzymatic antioxidant defenses, such as superoxide dismutase (SOD) and catalase (CAT) activity, was observed. The injection of NAC (150 mg/kg i.p. at 30 min after LPS injection) decreased the ROS, the TNFalpha the MDA levels, iNOS expression and the GSSG/GSH ratio, and increased the antioxidant defenses in both macrophages and lymphocytes. Moreover, the NAC treatment prevented the activation of nuclear translocation of the nuclear factor kappaB (NF-kappaB), which regulates ROS, inflammatory cytokines and antioxidant levels. Our present results provide evidence that both cell types have a relevant role in the pathogenesis of endotoxic shock, and that NAC, by improving the redox state of these immune cells, could increase mouse survival. Thus, antioxidants could offer an alternative treatment of human endotoxic shock.