The occurrence of Haemophilus in nasopharyngeal material from 130 hospitalized children aged 0—7 years has been investigated. If a selective medium was used, the carrier rates were very close to 100 per cent. The lowest rate was found in children aged less than one year (78 per cent). Out of a total of 137 Haemophilus strains isolated, H. influenzae amounted to 49 per cent, the remaining 51 per cent were shared between the species H. parainfluenzae, H. haemolyticus and H. parahaemolyticus. V‐factor independent strains were rarely found. The methods used for the determination of the growth‐factor requirements and their importance for the classification of the bacterial strains are discussed. An unclarified but constant finding of antibodies to H. influenzae type d antigen in sera from children aged more than nine months is described. Such antibodies were not found in children below that age, and were rather infrequent in sera from a group of young adults. No correlation was found between the bacteriological findings, the clinical findings and the presence of antibodies to the six H. influenzae types in sera from the children investigated. Thus the investigation does not contribute to the understanding of the clinical significance of any of the Haemophilus species.
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