Serum and sputum concentrations of ampicillin or amoxycillin were measured in patients admitted to hospital for acute exacerbations of chronic bronchitis with purulent sputum. Mean peak serum levels of nearly 12 mg/l were found after 1600 mg bacampicillin (mean peak level in sputum 0.85 mg/l). The serum and sputum concentrations after 750 mg amoxycillin and 800 mg bacampicillin were comparable (mean peak serum levels approximately 9.5 mg/l, sputum concentrations 0.4 to 0.5 mg/l) although the drugs were not given in equimolar doses. Results after 1000 mg ampicillin by mouth were less satisfactory (mean peak serum level 7.8 mg/l) and only 0.25 mg/l was attained in the sputum. Minimum inhibitory concentrations of ampicillin and amoxycillin were measured for 177 Haemophilus influenzae strains. Most of the ampicillin MIC values were between 0.125 and 0.5 mg/l but more of the strains required 0.5 mg/l of amoxycillin. The amoxycillin MIC values were often one or two dilutions higher than those of ampicillin (p less than 0.001).