Fura Red is a visible light-excitable fura-2 analog that offers unique possibilities for ratiometric measurement of calcium ion in single cells by microphotometry, imaging or flow cytometry when used with single excitation, green-fluorescent calcium indicators. Fura Red AM is the cell-permeable version of Fura Red used for noninvasive intracellular loading. Fura Red AM can be simultaneously loaded into cells with Fluo-3 AM, Fluo-8® AM or Cal-520® AM. An advantage of combining two calcium dyes is that dyes with longer excitation wavelengths can be used. This usually causes less harm to the cells than using ratiometric dyes that are excited with UV- or near UV-light (e.g. Fura-2), as light at visible wavelengths is less phototoxic.