A new sensitive and rapid capillary electrophoresis (CE) assay for measuring reduced and oxidized thiols in human plasma has been developed. To prevent oxidation of the thiols, whole blood was immediately centrifuged after collection and the plasma proteins were precipitated with perchloric acid. The reduced thiols in the supernatant were derivatized quantitatively at 25 degrees C, pH 7.5 with a fluorescent reagent, fluorescein-5-maleimide (FM). The total plasma concentration of thiols, including the fraction coupled to proteins, was assayed after an initial reduction of the disulfide linkage in plasma with dithiothreitol. The separation of FM-thiols was performed in an acetonitrile/10 mM sodium phosphate-50 mM SDS buffer [25:75 (v/v); pH 7.0] using a fused-silica capillary (57 cm x 75 microm I.D.) at 45 degrees C. A 3-mW argon-ion laser (lambda(ex) 488 nm/lambda(em) 520 nm) was employed for FM-thiol detection. With the electric field of 530 V/cm, the time needed for the separation of FM-homocysteine, FM-glutathione and FM-N-acetylcysteine was less than 8 min. The lower limit of detection was 3 microM for the total thiols and 10 nM for the reduced thiols. The method was applied to, the determination of homocysteine levels in plasma from patients with end-stage renal disease.