Calcium measurement is critical for numerous biological investigations. Fluorescent probes that show spectral responses upon binding Ca2+ have enabled researchers to investigate changes in intracellular free Ca2+ concentrations by using fluorescence microscopy, flow cytometry, fluorescence spectroscopy and fluorescence microplate readers. The long-wavelength Rhod-2 Ca2+ indicators are valuable alternatives to Fluo-3 for experiments in cells and tissues that have high levels of autofluorescence. Rhod-2 AM is cell-permeable version of Rhod-2.