The colonisation rate of Branhamella catarrhalis in patients from 0 to 45 years of age was examined. Of 561 women admitted to hospital in labour, 6 (1%) carried B. catarrhalis in their throats but none carried the organism in their vaginas. None of 534 newborn babies became colonised at birth or during their 5 days' stay in hospital. Neither were 102 neonates < 1 month of age in hospital colonised. The maximum colonisation rate during childhood was observed in children 1-48 months of age with 143 of 266 (54%) children colonised. Among children 4-15 years of age, four of 57 (7%) children with healthy respiratory tracts were colonised. Significantly more children with upper or lower respiratory tract infections (RTI) were colonised (68%) than were children without such infections (36%), (P < 0.001). After recovery from RTI, the isolation rate in the RTI group fell to that of the non-RTI group. A seasonal variation in prevalence was not observed. Of all the strains of B. catarrhalis isolated, 84% produced beta-lactamase.