The results of antibiotic therapy in 271 patients suffering from acute exacerbations of chronic bronchitis are presented. The effectiveness of the better absorbed ampicillin esters (pivampicillin and bacampicillin) is confirmed, but side-effects from the pivampicillin present problems whereas bacampicillin is excellently tolerated, even in twice daily doses of 1600 mg. Amoxycillin, if given in 750 mg doses three times daily by mouth, is also safe and effective againstHaemophilus influenzae. However, if accurate MIC results are not available for both ampicillin and amoxycillin, the lesser degree of sensitivity to amoxycillin suggests that use of an ampicillin ester (such as bacampicillin) is to be preferred. Prophylactic use of antibiotics in chronic bronchitis patients does not seem logical to us.