A variety of mercaptans, especially the beta-mercaptocarboxylic acids, inhibited the selenium (Se)-dependent glutathione peroxidase (SeGSHpx) activity of chick liver postmitochondrial supernatant or cytosol and of purified bovine erythrocyte SeGSHpx. The effects of mercaptans and a glutathione analogue on Se utilization were determined by subcutaneous injection of test compounds into vitamin E-deficient chicks fed diets containing 0.1 ppm Se (as Na2SeO3) at times when greater than 50% of vitamin E- and Se-deficient chicks showed the vitamin E-, Se-deficiency disease exudative diathesis (ED). D-(-)-Penicillamine hydrochloride (the positive control model compound), sodium beta-mercaptopyruvate, t-butyl mercaptan and S-methylglutathione (nonmercaptan glutathione analogue) decreased SeGSHpx activity in chick liver postmitchondrial supernatants within 24 h of injection and increased the incidence of ED within 4 d. Other mercaptans tested did not increase ED incidence or affect liver, kidney or plasma SeGSHpx activities. Although mercaptosuccinic acid, N-(2-mercaptopropionyl)glycine and sodium thioglycolate each strongly inhibited SeGSHpx in vitro, each also significantly increased chick mortality in the dosage range tested; therefore, their effects on SeGSHpx in vivo could not be evaluated. It appears that the beta-mercaptocarboxylic acids, mercaptans with a high degree of steric hinderance in close proximity to the thiol group and a close structural analogue of glutathione are capable of altering selenium status in chicks.