A sensitive and specific reversed-phase high-performance liquid chromatography (HPLC) method is described for the quantitative determination of albendazole sulfoxide (ASOX); since albendazole sulfone (ASON) appears only in small amounts and albendazole (ABZ) normally does not appear in human plasma, only a qualitative determination of ASON and ABZ was made in human plasma. Plasma samples were extracted three times using ethylacetate and petroleum benzine; this yielded optically clear samples which after evaporation were dissolved in the HPLC solvent and injected onto an RP-C18 column, with ultraviolet detection at 290 nm. The detection limit of the main metabolite ASOX was 50 nM and that of ASON was 100 nM. The intraday coefficient of variation for ASOX was 3.3% at a concentration of 2.2 microM, and the interday coefficients of variation were 14.5, 7.3, and 9.1% at ASOX concentrations of 0.5, 2.5, and 5.0 microM, respectively. Calibration was linear in a concentration range of 0.05-12 microM for ASOX and 0.1-8 microM for ASON, respectively. Pharmacokinetic data of a patient with echinococcosis are presented.