Cyclopenin is one of an unusual family of benzodiazepine metabolites produced by a number of Penicllium species. Often mistakenly referred to as a mycotoxin, cyclopenin shows little toxicity against mammalian cells, bacteria or fungi in vitro. Cyclopenin is an intermediate in the biosynthesis of 3-O-methyl viridicatin, a strong inhibitor of the tumor necrosis factor alpha (TNF)-induced replication of human immunodeficiency virus (HIV). Lack of availability has hitherto restricted a more intensive investigation of this interesting metabolite.