During the initial clinical evaluation of a new aminopenicillin, bacampicillin, 21 patients were treated for acute exacerbations of chronic bronchitis. The study was “single-blind” and comprised three dose-levels, which permitted a systematic study of dose-response relationships. Both the ampicillin levels in serum and sputum and the clinical outcome were dose-dependent. Sputum purulence and ampicillin levels were found to be a significant factor for predicting clinical outcome. With 400 to 800 mg bacampicillin t.i.d. mean ampicillin levels inhibitory for Haemophilus influenzae were maintained in sputum for at least 6 to 8 h and the clinical results were satisfactory, whereas this time was considerably shorter with the 200 mg t.i.d. dose, which also gave poorer clinical results. The ampicillin sputum to serum level ratio was 13% at I h and 20 % at 3 h after a dose. No sideeffects were observed.