Bronchial provocation tests (PT) were performed with a purified and standardized extract of the house dust mite Dermatophagoides pteronyssinus (D. ptero.) in an attempt to determine the most sensitive criteria of bronchospasm. Nineteen asthmatic patients with skin test and RAST positive for D. ptero. had a positive bronchial PT. More than 40% increase of Raw insp. simultaneously with dyspnea or wheezing was found in each patient, whereas only 6/19 had a 20% increase of FEV1. 47% of the patients had a slight to moderate late phase of bronchial obstruction. None of the 9 asthmatic patients with skin test and RAST negative for mites had bronchospasm. It is concluded that a 40% in Raw insp., combined with clinical signs of bronchospasm, is a sensitive, specific and safe method of determining the threshold of bronchial sensitivity to a standardized allergen.