The fungal metabolite gliotoxin is characterized by an internal disulfide bridge and can exist in either disulfide or dithiol forms. Gliotoxin and other members of the epipolythiodioxopiperazine class of toxins have immunosuppressive properties and have been implicated in human and animal mycotoxicoses. The bridged disulfide moiety is thought to be generally essential for biological activity. Here we show that only the natural (oxidized) form of gliotoxin is actively concentrated in a cell line in a glutathione-dependent manner. Intracellular levels of the toxin can be up to 1500-fold greater than the applied concentration, and toxin in the cells exists almost exclusively in the reduced form. A simple model of toxin entry followed by reduction to the cell-impermeant dithiol explains active uptake, cell density dependence of EC50 values and predicts a value for the maximum concentration of toxin at limiting cell density in agreement with the experiment. Oxidation of the intracellular toxin results in rapid efflux from the cell that also occurs when glutathione levels fall following induction of apoptotic cell death by the toxin. This mechanism allows for minimal production of the toxin while enabling maximal intracellular concentration and thus maximal efficacy of killing in a competitor organism initially present at low cell density. The toxin effluxes from the apoptotic cell exclusively in the oxidized form and can further enter and kill neighboring cells, thus acting in a pseudocatalytic way.