Langerhans cells (LC) are dendritic antigen presenting cells of bone marrow origin which reside in the suprabasal layer of the epidermis. They express high concentrations of Class II MHC glycoproteins on their plasma membrane and transport cutaneous antigen to local lymph nodes for presentation to helper T cells. They are thus essential for the induction of cutaneous immunity. Gliotoxin is a member of the epipolythiodioxopiperazine (ETP) group of fungal metabolites, derived from the human pathogen Aspergillus fumigatus. It has been shown to have immunomodulating properties in vivo and in vitro, and has been proposed as a potential immunosuppressant for transplantation therapy. Epicutaneous application of gliotoxin reduced the numbers of epidermal LC by 30-35 per cent with an associated morphological change from highly dendritic to a more rounded form. Electron microscopic studies showed selective damage to LC at very low (nM) concentrations of gliotoxin, with no obvious effect on adjacent keratinocytes. LC numbers remained depleted for 13 weeks after initial treatment, suggesting that systemic suppression or prolonged retention of gliotoxin within the skin may play a role in its mechanism of action.