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.