A sensitive high-performance liquid chromatographic (HPLC) assay for N-acetylcysteine in human plasma and urine has been developed. The method employs a prechromatographic stage to produce the dinitrophenyl derivative of N-acetylcysteine, which is chromatographed on a 5-microns C18 bonded reverse-phase column using an external standard method. The derivatized N-acetylcysteine molecule has a retention time of congruent to 13 min at a flow rate of 1.0 mL/min for a mobile phase of methanol-aqueous 0.05 M citrate-0.001 M EDTA buffer pH 7.0 (30:70). In a multistep extraction procedure, which is slightly modified for the N-acetylcysteine assay in urine, the limits of sensitivity for the plasma and urine assays were found to be congruent to 60 ng/mL and congruent to 200 micrograms/mL, respectively. Preliminary data from pilot studies in human subjects are presented.