Six patients with allergic asthma were studied in bronchial challenge tests with their specific antigen. In five cases 71oxo-7-thiomethoxyxanthone-2-carboxylic acid, sodium salt (OTMX), a new xanthone with mediator-release inhibitory properties, given as an inhaled solution under ‘single-blind’ conditions, protected against the immediate-type bronchospastic response. This protection was statistically significant (p = 0.016) at the highest dose level used. Late reactions which occurred in two patients were unaffected by the compound.