The interaction between inhaled 1,2-dichloroethane (ethylene dichloride; EDC) and dietary Disulfiram (tetraethylthiuram disulfide; Antabuse; DSF) was investigated for male Sprague-Dawley rats in terms of urinary levels of thio-compounds extractable in ethyl acetate and then hydrolyzed in alkali (the classic urinary thioether assay). The assay was found to be an inadequate biological monitoring indicator for EDC or DSF exposure during the DSF/EDC interaction at exposures of 0, 153, 304 and 455 ppm EDC (7 hr/day, 5 days/week, 30 exposure days) for rats fed with AIN-76 diet fortified with 0.15% DSF. EDC inhibited the excretion of DSF-derived thio-compounds with increasing EDC concentration; the thioether content was dose-related in the absence of DSF. In situations where confounding agents generate neutral S-containing urinary metabolites without involvement of endogenous glutathione, the classic thioether assay requires supplementation by other biochemical monitoring strategies.