– Fluo-3 (ammonium salt) is a fluorescent calcium indicator commonly used in flow cytometry and cell-based experiments to detect changes in intracellular calcium levels. [1] Its absorption maximum at 506 nm makes it compatible with excitation at 488 nm by argon-ion laser sources. Fluo-3 provides intense fluorescence upon binding calcium, detected at a maximum emission at 526 nm which can be monitored by FL1 (green, 525 nm band pass) sensors in flow cytometry.