SummaryThein vitroactivity of ampicillin, amoxicillin/clavulanate,Cefadroxil, cefaclor, cefuroximeCaxetil), co-trimoxazole, doxycycline, ciprofloxacin, ofloxacin, erythromycin, and roxithromycin was tested against unselected isolates ofS. pneumoniae(70),H. influenzae(93), andM. catarrhalis(46), cultured from clinically significant sputum samples of general Practice patients. All isolates of S. pneumonia were highly susceptible to ampicillin;Cefadroxiland cefaclor were markedly less active on a height basis; resistance was only observed with co-trimoxazole (4.3%), doxycycline (5.7%), and erythromycin (2.9%); however, ciprofloxacin and ofloxacin showed median MICs (MIC50), that were only one dilution below breakpoint,ß-lactamase was detected in 14.0% ofH. influenzaefolates; all isolates weresusceptibleto amoxicillin/clavulanate, cefaclor, and cefuroxime (axetil), although MICs were generally higher or cefaclor; the highest activity was exhibited by ciprofloxacin and ofloxacin; apart fromCefadroxil, erythromycin, and roxithromycin, that showed only marginal activity, resistance was observed with co-trimoxazole (4.3%) and doxycycline (1.1 %). All (including 71.7% ofß-lactamase producing) isolates ofM. catarrhaliswere susceptible to amoxicillin/clavulanate, cefaclor and cefuroxime (axetil), although MICs were markedly lower for amoxicillin/clavulanate; ciprofloxacin and ofloxacin showed the lowest MICs; resistance was only observed with Cefadroxil (2.2%). In conclusion, the antimicrobial agents showing the most uniformely highin vitroactivity against the 3 common community respiratory pathogens tested in the present study, were amoxicillin/clavulanate and, to a lesser extent, cefuroxime (axetil).