Aminophenyl fluorescein (APF, A36003) and hydroxyphenyl fluorescein (HPF), two reactive oxygen species (ROS) indicators developed by T. Nagano, offer greater specificity and stability than dichlorodihydrofluorescein diacetate (H2DCFDA, D399). APF and HPF show limited non-selective reactivity and relatively high resistance to light-induced oxidation. Both of these new fluorescein derivatives are nonfluorescent until they react with the hydroxyl radical or peroxynitrite anion; APF will also react with the hypochlorite anion. Together, these two ROS indicators can selectively detect hypochlorite anion. Upon oxidation, both APF and HPF exhibit bright green fluorescence (excitation/ emission maxima ~490/515 nm), making them compatible with any instrument that can detect fluorescein.