2',7'-bis(2-Carboxyethyl)-5(6)-carboxyfluorescein (BCECF) is a widely used fluorescent indicator for the measurement of cytoplasmic pH in cells. BCECF has been used to: observe regulation of basal acid secretion by glycogen synthase kinase 3, identify enzymatic activity, determine transport activity in human erythrocytes, observe Na+/H+ exchange activity and regulation of gastric acid release.