Fura Red, tetrapotassium salt is a visible light—excitable fura-2 analog that offers unique possibilities for ratiometric measurement of Ca²⁺ in single cells by microphotometry, imaging or flow cytometry when used with single excitation, green-fluorescent calcium indicators. This water-soluble salt form is useful for intracellular loading by microinjection, infusion from patch pipette or uptake induced by our Influx pinocytic cell-loading reagent (I-14402).