Objective: We report on the effect of glutathione, an antioxidant compound on the airway response to the ultrasonically nebulised distilled water (UNDW, 'fog') challenge. Methods: 12 subjects with mild-to-moderate bronchial asthma underwent double-blind, cross-over pretreatment, administered 30 min earlier, in a randomised order with inhaled glutathione (G) (600 mg), sodium cromoglycate (SCG) (20 mg) and placebo (P), followed by the challenge. Results: After P pretreatment UNDW challenge caused a mean 20.41% decrease in FEV-1 (p < 0.05), after G, a mean 6.04% fall in FEV-1 (p = n.s.), and after SCG a mean 5.99% fall in FEV-1 (p = n.s.). Conclusions: G significantly attenuated 'fog'-induced falls in FEV-1 (p < 0.001 compared with P) and showed a protective effect on UNDW-induced bronchoconstriction. Copyright (C) 1999 S. Karger AG, Basel.