The authors have made a systematic study by PRICK-test and RAST, sometimes followed by a rhino-manometric provocation, of the frequency of sensitization to the two moulds Alternaria and Cladosporium in 200 "all comers" children in a paediatric consultation. The frequency was low, only 10% for those consulting in the groups of asthma, rhinitis, pollinosis, repeated respiratory infections and syndromes of obstructed expiration taken together, and 13% for allergic asthma only. This sensitivity is rarely isolated, but is most often accompanied, or complicated by, other pneumoallergens, such as dust, mites, pollens and sometimes cockroaches. It imposes certain particular clinical traits on asthma (summer asthma or recurring asthma) and in others appears to be strongly influenced by the habitat. The skin tests seem to be very weak and RAST insufficient to be sure of real sensitization. Diagnosis, always difficult, is probably best done by a provocation test (preferably rhino-manometry) and this jumps over the bundle of clinical and biological arguments, as well as being, at least in our country, the most useful mimic of atmospheric moulds. The authors insist on the importance of the interrogation which should be concerned with the chronological and environmental circumstances and all respiratory infections that are suspected of having an allergic factor.