The Ca²⁺ ionophore 4-bromo A-23187 is commonly used for in situ calibrations of fluorescent Ca²⁺ indicators, to equilibrate intracellular and extracellular Ca²⁺ concentrations and to permit Mn²⁺ to enter the cell to quench intracellular dye fluorescence. Unlike A-23187, brominated A-23187 is essentially nonfluorescent and can be used with UV-excitable calcium indicators like fura-2, indo-1 and quin-2 as well as the visible light-excitable indicators, including fluo-4, fluo-3, Calcium Green and rhod-2. 4-bromo A-23187 can also be used to equilibrate extracelular and intrcellular Mg²⁺ concentrations, making it useful for calibrating Mg²⁺ indicators.