A fluorogenic substrate for carboxylesterase and AChE; upon hydrolysis by carboxylic esterase or AChE, 4-MU is released, which displays pH-dependent excitation maxima of 330, 370, and 385 nm at pH 4.6, 7.4, and 10.4, respectively, and an emission maximum between 445-454 nm; has been used to detect the presence of carboxylesterases in isolated human lung tissue