The in vitro antibacterial activity of ciprofloxacin, a new quinoline carboxylic acid, was tested against 1671 recently clinically isolated bacterial strains, by measuring the minimum inhibitory concentrations (MIC). Comparisons were made with other quinolones: nalidixic acid, norfloxacin, and other drugs: piperacillin, cefoxitin, cefotetan, ceftazidime, tobramycin, rifampin, tetracycline, chloramphenicol. Ciprofloxacin was very active against the tested species and was the most active drug against all the bacterial strains, with a geometric mean, a MIC50 and MIC90 of 0.27, 0.12 and 2 micrograms/ml, respectively.