A fluorescent compound that binds Ca++ tightly. The wavelengths of light that cause fluorescence when Ca++ is bound are longer than the wavelengths that cause fluorescence when Ca++ is not bound. When excited at two different wavelengths, the ratio of the fluorescence intensities at the two wavelengths gives the ratio of the concentrations of bound to free Ca++. Free quin-2 concentration can be measured precisely, so free Ca++ concentration can be calculated precisely. Quin-2 may be injected into cells to measure moment-to-moment changes in intracellular Ca++ concentration.