Gliotoxin is a toxic metabolite of Aspergillus fumigatus Fresenius and other fungi. It has been suggested that this toxin may play an important role in the pathogenesis of aspergillosis as gliotoxin has immunosuppressive activity both in vitro and in vivo. We have determined the optimum growth conditions for the production of gliotoxin by selected isolates of A. fumigatus using a number of defined media. Gliotoxin was detected by thin layer chromatography and high performance liquid chromatography. The carbohydrate source, concentration of carbohydrate in the growth medium and incubation temperature were all found to influence gliotoxin production. Optimum growth conditions for gliotoxin production in our study were Czapek-Dox broth containing 30% glucose and incubation at 37°C. Most of the gliotoxin was produced after 29 h incubation, during the exponential phase of growth. A novel method for screening large numbers of A. fumigatus isolates for gliotoxin production, which is both quick and easy, has also been developed, based on the ability of gliotoxin to inhibit the adherence of lung fibroblast (L929) cells to plastic microtitre plates.