Alterations in the cellular chemical environment, such as changes in pH, changes in ion concentrations, and changes in the oxidative state, are associated with both ongoing physiological activity and pathophysiological processes. This research focused on how such changes in the cellular chemical environment affect the activation of matriptase, a membrane-bound serine protease. Manipulating the cellular chemical environment of mammary epithelial cells revealed that the activation of matriptase from a zymogen to an active enzyme can be rapidly induced by a decreased pH level, or an increased oxidative state. Changing the pH from a physiological level (pH 7.4) to a mildly acidic level (pH 6.0) accelerated matriptase activation by shortening the time for the onset of activation from around 30-40 minutes to 2-3 minutes. Matriptase activation was significantly attenuated by the reductive state of the cytosol. These mechanisms likely prevent premature matriptase activation when trafficking through the increasingly acidified secretory pathway en route to the cell membrane. Similar attenuation was seen in the presence of extracellular physiological concentrations of chloride ions. This mechanism aids in matriptase regulation at the cell surface. The control of matriptase activation by acidity and redox state gives matriptase-expressing cells the ability to activate matriptase proteolysis when the cellular chemical environment changes. Shift to a more acidic and oxidized chemical environment are seen during stressful cellular events such as hypoxia and apoptosis. Therefore, it was hypothesized that these cellular events would induce matriptase activation. Cobalt chloride (CoCl2)-induced hypoxia and doxorubicin-mediated apoptosis each lead to increased matriptase activation. This activation was significantly suppressed by the antioxidant N-acetylcysteine (NAC), which further confirmed the stimulating role of an oxidized state in matriptase activation. The unique regulation of matriptase autoactivation by acidity and redox state makes matriptase a new candidate sensor for a variety of cellular processes under both physiologic and pathophysiological conditions.