The antiallergic properties of a new coumarin compound (BM 15.100) were tested in two separate trials, using bronchial provocation tests in asymptomatic volunteers with extrinsic allergic bronchial asthma. Study I was a blind study in 10 subjects with placebo controls and three additional non-medicated controls. Study II was an open study with non-medicated controls. Body plethysmography (study I) and spirometry (study II) were used for the assessment of bronchial obstruction. A significant protective effect of a single oral 20 mg dose of BM 15.100 administered 60 min prior to allergen inhalation was found in both trials. The effect of 10 mg BM 15.000 was not significant. Skin test, pulse rate, and diastolic blood pressure were not influenced. The results indicate a possible therapeutical value of BM 15.100.