It is well established that H2O2 is a cytotoxic agent but evidence also suggests that H2O2 may be an important regulator of eukaryotic signal transduction. Cayman's H2O2 Cell-Based Assay provides a simple fluorometric method for the sensitive quantitation of H2O2 in cultured cells. H2O2 is detected using 10-acetyl-3,7-dihydroxyphenoxazine (ADHP), a highly sensitive and stable probe for H2O2. In the presence of horseradish peroxidase, ADHP reacts with H2O2 with a 1:1 stoichiometry to produce highly fluorescent resorufin (excitation = 530-560 nm; emission = 590 nm). Catalase, an H2O2 scavenger, is included in the kit for checking specificity of the assay.