Blood platelets are central to haemostasis, and reactions in platelets involving sulfhydryl groups play important roles in platelet function. Reduced glutathione (GSH) plays an important role in platelet aggregation and glutathione-depleting chemicals inhibit platelet aggregation. The lipophilic drug disulfiram, because of its affinity for sulfhydryl groups, is a highly thiol-reacting agent. As a consequence, GSH and sulfhydryl groups of protein cysteines in human platelets, in analogy to other components of human blood, are a potential target of disulfiram. In the present study, we have shown that exposure of human platelets to disulfiram causes the depletion of platelet GSH and augmentation of mixed disulfides between GSH and protein sulfhydryl groups to form protein-glutathione mixed disulfides (S-glutathionylated proteins). The depletion of platelet GSH and the increase in S-glutathionylated proteins occurred at concentrations of disulfiram that inhibited platelet aggregation, suggesting that protein S-glutathionylation is involved in the inhibition of platelet aggregation caused by disulfiram.