The sensitivity to chloramphenicol and ampicillin of 127 strains of H. influenzae was tested. One hundred and one strains were obtained from the pharyngeal exudate of 828 healthy carriers under 5 years of age and 26 from the spinal fluid of children with meningoencephalitis. Sixty per cent of isolations corresponded to type b; 40 per cent were non typical and we only identified one type e. All H. influenzae obtained from spinal fluid corresponded to type b. The frequency of healthy carriers was greater in intrafamily contacts of children with H. influenza meningoencephalitis (p less than 0.01). Percentages of resistance to ampicillin varied between 6 and 23% for the different groups; we found no statistical difference among them (p less than 0.2). The prevalence of H. influenza resistant to ampicillin in the population studied was 1.2% for type b strains and 0.2% for non typical bacteria. Fourteen per cent of penicillin resistant type b strains were identified; all 127 isolations were to chloramphenicol; therefore, we recommend this drug instead of ampicillin for the treatment of H. influenza infections, with the exception of acute otitis media.