The comparative activity of ciprofloxacin against 272 clinical isolates and reference strains of anaerobes was determined by an agar dilution method. The majority of strains of Bacteroides fragilis and clostridia were resistant (MIC greater than 2 mg/l). The effect of pH, inoculum size and medium composition was also studied; the activity of ciprofloxacin was decreased at acid pH and by increasing inoculum size but was unaffected by the composition of the test media. Ciprofloxacin should not be used for the treatment of anaerobic infections because of its limited in-vitro activity against these organisms.