The biology of highly reactive oxygen radical species is of great interest in many biomedical research disciplines, including, neurodegeneration and aging, cancer, and infectious diseases. There are a number of fluorescent reagents that can be used to detect free radicals, such as 2,7-dichlorodihydrofluorescein (DCDHF), but they have significant limitations due to their facile oxidation by light and numerous non-radical oxidants such as hydrogen peroxide (H2O2). APF is an aromatic amino-fluorescein derivative that has little intrinsic fluorescence. It undergoes oxidation only by the hydroxyl radical, hypochlorite ion, and certain peroxidase intermediates, but is inert to NO, H2O2, superoxide, and other oxidants. Upon oxidation, APF is converted to the highly fluorescent molecule fluorescein, allowing the simple direct detection of highly reactive biological radicals.