The diagnostic value of a purified and standardized extract of the house dust mite Dermatophagoides pteronyssinus (Pharmalgen, Pharmacia) has been tested by nasal or bronchial allergic provocation tests (PT). 50 patients (31 with rhinitis and 19 with asthma) who had a skin test and RAST positive for D. ptero. were tested: all had a positive PT. 47% of the patients with asthma had a late phase of bronchial obstruction. PT were negative in 18 subjects not allergic to D. ptero. It is concluded that, although specific, PT are not necessary if the diagnosis has already been established by the association of a positive skin test and RAST.