A regimen of cefprozil (500 mg twice daily), a new oral cephalosporin with a broad in vitro spectrum of antimicrobial activity, was compared to standard regimens of cefaclor (500 mg three times daily), cefuroxime axetil (500 mg twice daily), or amoxicillin/clavulanate (500 mg/125 mg three times daily) for the treatment of lower respiratory tract infections (mainly bronchitis and acute exacerbations of chronic bronchitis) in adults in three open-label, randomized trials. In the first trial, in which bacterial pathogens were isolated in initial cultures for only one-third of the patients, 90% of the pathogens were susceptible to cefprozil. A satisfactory clinical response was noted for 84% of the evaluable patients who received cefprozil versus 79% of those who received cefaclor for treatment of lower respiratory tract infections; rates of bacteriologic efficacy were 82% and 78%, respectively. In the second study rates of satisfactory clinical response were 96% with cefprozil and 83% with cefuroxime axetil (P less than .03) for treatment of bronchitis; the respective bacteriologic response rates were 100% and 92%. In the third trial, clinical efficacy was 91% for cefprozil and 87% for amoxicillin/clavulanate for treatment of bronchitis; bacteriologic efficacy was 95% and 96%, respectively. Tolerability and safety profiles were comparable, except that there was a higher rate of diarrhea among patients who received amoxicillin/clavulanate (P = .03).