Human peripheral T cells are considered to be easily susceptible to oxidative stress because these cells lack peroxidase activity. Therefore, in a previous study, we investigated the site of ROS formation by utilizing Mito-Capture, H(2)DCFDA (succinimidyl ester of dichlorodihydrofluorescein diacetate), DAPI (4',6-diamidino-2-phenylindole), and LysoSensor. Our results showed that ROS formation was apparently diffusely distributed in T cells oxidatively stressed with 0.1 mM hydrogen peroxide. Moreover, lysosomal swelling and deformity, possibly revealing lysosomal membrane destabilization, were observed in these cells. Based on the above-mentioned results, we concluded that an apoptotic cascade involving early lysosomal membrane destabilization exists in the hydrogen peroxide-induced apoptosis of human peripheral T cells. Therefore, the possible involvement of lysosomal protease leakage caused by hydroxyl radical formation in lysosomes (possibly resulting in mitochondrial membrane dysfunction) is considered to play an important role in hydrogen peroxide-induced T cell apoptosis. Hydrogen peroxide-mediated destabilization of lysosomal membranes with release of hydrolytic enzymes such as many kinds of cathepsins into the cell cytoplasm can lead to a cascade eventuating in cell death. To assess the importance of the intralysosomal pool of redox-active iron, we examined the effect of blockade of lysosomal digestion by exposing T cells to the lysosomotropic alkalinizing agent ammonium chloride (NH(4)Cl). Preincubation of human peripheral T cells with 10 mM NH(4)Cl for 4 h dramatically decreased apoptotic death caused by subsequent exposure to hydrogen peroxide (H(2)O(2)), and lysosomes and mitochondria showed almost normally preserved appearance. Therefore, we concluded here that lysosomal protease leakage caused by hydrogen peroxide in T cells was prevented by preincubation with ammonium chloride (NH(4)Cl).