A total of 43 hospitalized adult patients with acute exacerbations of chronic bronchitis or bronchiectasis due to Gram-negative bacteria were randomized to receive either enoxacin (400 mg bd) or amoxycillin (1,000 mg tid) for 7-12 days. Micro-organisms isolated included 24 Haemophilus influenzae (three beta-lactamases positive), 11 Branhamella catarrhalis (six beta-lactamase positive), two Pseudomonas aeruginosa and two Neisseria meningitidis in 37 evaluable patients. In the enoxacin group (23 patients) 82.6% of the patients were clinically cured or improved against 93% of patients in the amoxycillin group (14 patients). In the enoxacin group 76% of the pathogens were eradicated with two failures (P. aeruginosa), one relapse (H. influenzae) and three superinfections (Streptococcus pneumoniae). In the amoxycillin group, 71% of the pathogens were eradicated with 29% relapses. The differences between the two groups were not statistically significant. An increase in theophylline concentration occurred in 15 of 16 patients receiving simultaneous administration of theophylline, without clinical evidence of toxicity when theophylline dosage was reduced and enoxacin continued. Enoxacin appears to be as effective as amoxycillin in the treatment of acute exacerbations of chronic bronchitis due to susceptible Gram-negative bacteria.