The fungistatic and fungicidal activity of N-chlorotaurine (NCT), a long-lived oxidant produced by stimulated neutrophils, was investigated. Physiological concentrations (75-100 mu M) of NCT showed clear fungicidal activity against a range of Aspergillus isolates. Moreover, killing by NCT was significantly increased in the presence of ammonium chloride, explained by the formation of monochloramine by halogenation of ammonium. One clinical isolate of Aspergillus fumigatus was characterized for the production of the immunosuppressive agent gliotoxin, and NCT was shown to cause destruction of gliotoxin, possibly via reduction of the disulphide bridge. Because of its endogenous nature and its high antifungal activity, NCT appears to be a good choice for topical treatment of Aspergillus infections, and the results of this study further substantiate its therapeutic efficacy.