The fungal metabolite gliotoxin shows selective toxicity to cells of the immune system and has been implicated in the aetiology of invasive aspergillosis. The related toxin sporidesmin is the causative agent of facial eczema in sheep. The toxicity of these compounds has been related to their ability to redox cycle intracellularly and thus produce damaging free radicals. These toxins are also potentially capable of forming mixed disulphides with thiol groups on proteins by virtue of their bridged disulphide structure. We show here that gliotoxin can inactivate horse liver alcohol dehydrogenase by either oxidative damage or covalent modification of thiol groups on the enzyme. Either Cys-281 or Cys-282 is selectively modified. Neither of these residues are at the active site. Covalent modification occurs in the absence of reducing agents such as dithiothreitol. In the presence of dithiothreitol no protection is observed and the rate of inactivation is enhanced although as expected no covalent modification occurs. Gliotoxin can therefore inhibit alcohol dehydrogenase by either pathway and this will depend on the availability of reducing agents such as glutathione and/or how readily the reactive oxygen species generated are removed.