Mitochondria are major producers of cellular superoxide. The production of low to moderate levels of superoxide is critical for the proper regulation of many essential cellular processes including gene expression, signal transduction, and muscle adaptation to endurance exercise training. Uncontrolled mitochondrial superoxide production can trigger cellular oxidative damage that contributes to the pathogenesis of a wide variety of disorders including cancer, cardiovascular diseases, neurodegenerative diseases and aging. The detection of intracellular mitochondrial superoxide is of central importance to understanding proper cellular redox regulation and the impact of its dysregulation on various pathologies. Cell Meter™ Fluorimetric Mitochondrial Superoxide Activity Assay Kit uses our unique Superoxide Indicator to quantify superoxide level in live cells. MitoROS™ 580 is live-cell permeant and can rapidly and selectively target superoxide in mitochondria. It generates red fluorescence when it reacts with superoxide. The Cell Meter™ Fluorimetric Intracellular Superoxide Detection Kit provides a sensitive, one-step fluorimetric assay to detect mitochondrial superoxide in live cells with one hour incubation. This kit can be used for fluorescence microplate readers and fluorescence microscopy applications.