Cefaclor and tetracycline were compared in a single-blind study designed to treat patients with acute bacterial bronchitis and acute exacerbations of chronic bronchitis. Twenty-five pathogens (including 19 of Haemophilus influenzae and four of Streptococcus pneumoniae) were obtained from sputum samples of 48 patients. No pathogen could be cultured from the sputum of 23 patients. All of these pathogens were susceptible to cefaclor, while 12 (63%) of the 19 H influenzae isolates and three of the four S pneumoniae isolates were resistant to tetracycline. When the susceptibility of the 25 isolates to other commonly used antibacterials was tested, 18 isolates of H influenzae were resistant to erythromycin and one was resistant to ampicillin. (One H influenzae isolate was not tested for erythromycin susceptibility.) The four isolates of S pneumoniae were susceptible to erythromycin and ampicillin. Satisfactory results were achieved in 21 of the 23 patients receiving cefaclor. After four to six days of cefaclor therapy, the other two patients were diagnosed as having bronchopneumonia, and parenteral antibiotic therapy was instituted. Of the 25 patients assigned to the tetracycline regimen, three with resistant H influenzae had unsatisfactory clinical responses and required parenteral antibiotic therapy for recovery. Although patients were randomly assigned to therapy, only three of the 16 patients infected with tetracycline-resistant organisms were assigned to the tetracycline group, and all three failed to respond to treatment. Had the patients been more evenly distributed according to susceptibilities, it is possible that more treatment failures would have occurred in the group receiving tetracycline.