A liquid chromatography (LC) method for the simultaneous measurement of the main low molecular mass thiols (i.e., cysteine, cysteinylglycine, homocysteine, and glutathione) in human plasma is described. The sample treatment consists of the reduction of disulfide bounds with tri-n-butylphosphine and protein precipitation with trichloroacetic acid followed by precolumn derivatization with a thiol-selective fluorogenic reagent (7-fluoro-2,1,3-benzoxadiazole-4-sulfonamide). The structure of thiol derivatives is assessed using electrospray ionization-mass spectrometry (MS). The stability of resulting adducts in acidic medium (24 h at 10 degrees C) allows the automation of the technique and a high throughput of samples (approximately 50 per day). Separation is complete within 12 min using isocratic reversed-phase mode, and detection is operated by spectrofluorimetry (lambda ex = 385 nm and lambda em = 515 nm). Quantitation is performed by an internal standardization mode using thioglycolic acid. The LC method is fully validated, and homocysteine concentrations obtained in plasma samples are compared with values measured using either fluorescence polarization immunoassay or capillary gas chromatography-MS; a good correlation is observed between LC and both methods. The method has been applied in daily use to a large-scale study in a human healthy population, and some resulting data are discussed.