A new fluorescent probe 1,4-methylumbelliferyl-2',4',6'-trinitropheyl ether (Probe 1) was designed and synthesized. Probe 1 was a nonfluorescent compound and was synthesized via the one-step reaction of 4-methylumbelliferone (4-MU) with 1-chloro-2,4,6-trinitrobenzene. Upon mixing with biothiols under neutral aqueous conditions, the 2,4,6-trinitrophenyl group of 1 was efficiently removed, and the emissive free dye 4-MU was released, hence leading to a dramatic increase in fluorescence emission of the reaction mixture. A good linear relationship was obtained from 0.1 to 4.0 micromol L(-1) for cysteine (Cys), from 0.1 to 3.0 micromol L(-1) for homocysteine (Hcy), and from 0.2 to 3.0 micromol L(-1) for glutathione (GSH), respectively. The detection limits of Cys, Hcy, and GSH were 24.3, 35.6, and 26.8 nmol L(-1), respectively. Furthermore, probe 1 was highly selective for biothiols without the interference of some biologically relevant analytes and has been applied to detecting biothiols in human serum samples.