Membrane-permeant AM ester form of Fluo-3 that can be loaded into cells via incubation. Because of the relatively low water solubility of the AM ester, Pluronic F-127, a mild detergent, is often used as a dispersing agent to facilitate dye loading. Fluo-3 AM ester itself does not bind Ca2 , but it is readily hydrolyzed to Fluo-3 by endogenous esterases once the dye is inside the cells. An important application of Fluo-3 AM ester is its use in high throughput drug screening.