Dual-emission ratiometric measurement of lysosomal pH using LysoSensor™ Yellow/Blue dextran (Cat. no. L22460). MDCK cells labeled with the fluorescent dextran were exposed to pH-calibration buffers (pH 3.5 or pH 6.0) in the presence of nigericin (Cat. no. N1495) and monensin. Pseudocolored images were constructed from two emission images at 450 ± 33 nm and 510 ± 20 nm, both excited at 365 ± 8 nm. (Chem Biol 6, 411 (1999))