Ca2+-indicator fluo-3 for use with visible-light excitation sources in flow cytometry and confocal laser-scanning microscopy. Since about 1996, fluo-3 has also been extensively used in cell-based high-throughput screening assays for drug discovery. Fluo-3 AM is essentially nonfluorescent until it is hydrolyzed intracellularly and/or in the presence of Ca2+. The green-fluorescent emission (~525 nm) of Ca2+-bound fluo-3 is conventionally detected using optical filter sets designed for fluorescein (FITC). As prepared by Minta, Kao and Tsien, fluo-3 was originally reported to undergo an ~40-fold increase in fluorescence upon binding Ca2+.