AbstractOne hundred and nine subjects with chronic bronchitis were studied prior to winter and without clinical infection, to determine baseline patterns of bacterial colonisation. Qualitative analysis of cultures of oropharyngeal swabs showed little difference from age matched normal controls (17) except for growth of small numbers of Gram negative coliforms in the chronic bronchitic group. Quantitation of bacteria colonising the oropharynx showed small numbers (mean of 105cfu/ml), with no particular bacteria dominating.Haemophilus influenzaewas present in 7.3% of throat swabs from chronic bronchitic patients, but the organism was always less than 10% of the total count. Quantitation of bacteria in sputum showed significantly higher numbers (mean 107cfu/ml).H. influenzaewas detected in 25.7% of available specimens, and when present constituted>90% of the total count.Biotyping ofH. influenzaeisolates demonstrated a separate colonisation of the upper and lower respiratory tracts.