Fluorescent indicator dye which is primarily used for measuring changes in intracellular Ca2+. Increases in ion concn results in increased fluorescence without a shift in wavelengths. Prepn: R. Y. Tsien, Biochemistry 19, 2396 (1980). Use in measuring Ca2+ flow: J. N. Crofts, G. J. Barritt, Biochem. J. 264, 61 (1989); Zn2+-protein interactions: J. R. Jefferson et al., Anal. Biochem. 187, 328 (1990); in fluorescent lifetime imaging of cells: J. R. Lakowicz et al., Cell Calcium 15, 7 (1994). Fluorescent properties of acetoxymethyl ester: N. Miyoshi et al., Appl. Fluoresc. Technol. 4, 3 (1992). Dissociation parameters for Ca2+ and Mg2+ complexes: D. T. W. Bryant, Biochem. J. 226, 613 (1985); and photophysics: V. Van Den Bergh et al., Photochem. Photobiol. 61, 442 (1995). Review of use in measurement of Ca2+: R. Y. Tsien, T. Pozzan, Methods Enzymol. 172, 230-232 (1989).