Previous studies showed the inhibitory effects of crude ethanolic onion extracts (COE) on allergic skin reactions in man as well as on allergen-induced bronchial asthma in man and guinea-pigs. Work is in progress in order to identify both the mode of action of COE and the active substance(s). The present study describes asthma-protective effects of isothiocyanates. Groups of at least 5 guinea-pigs sensitized to ovalbumin were challenged twice (time 0 and 10 min) by the inhalation of ovalbumin 30 min after oral treatment with increasing doses of the agent tested or control solutions. Bronchial obstruction (BO) was measured by whole body plethysmography. Chloroform extracts of onions showed similar protective effects on BO as COE. The water-soluble fraction of COE was inactive. Benzyl-isothiocyanate (BITC) was identified as one component of onion lipids by combined gas chromatography/mass spectrometry. BITC inhibited BO in a dose-dependent fashion: 150 mg/kg: 89%; 75 mg/kg: 76%; 30 mg/kg: 66%; 15 mg/kg: 49%. Ethyl-isothiocyanate and allyl-isothiocyanate showed similar effects; p-hydroxy-benzyl-isothiocyanate, a very unstable mustard oil, was ineffective. Additional experiments showed no antagonistic effects of COE on histamine- or acetylcholine-induced BO. The antiasthmatic effects of onions and - perhaps - other plants may be mediated at least in part by isothiocyanates.