Gliotoxin is a potent epithiodioxopiperazine mycotoxin produced by species in the genera Gliocladium, Aspergillus and Penicillium. At the cellular level gliotoxin has been shown to inhibit a broad range of unrelated mechanisms, such as inhibition of chymotrypsin-like activity of the 20S proteasome, Ca2+ release from mitochondria, activation of transcription factor NF-κB in response to a variety of stimuli in T and B cells, anti-inflammatory activity and inhibition of farnesyltransferase and geranyl- geranyltransferase. The mode of action appears to be via covalent binding to proteins through mixed disulphide formation. Gliotoxin has been shown to inhibit a number of thiol-requiring enzymes and also displays antioxidant and immunomodulatory activity.