We tested ciprofloxacin, a new quinoline derivative, against 783 gram-positive and gram-negative bacteria utilizing either standardized microbroth or agar dilution methods. Ciprofloxacin activity was compared to that of cephalosporins, enoxacin, norfloxacin, ampicillin, chloramphenicol, tobramycin, ticarcillin, erythromycin and trimethoprim/sulfamethoxazole. MIC90 values (range) in micrograms/ml were: Pseudomonas aeruginosa = 1 (less than or equal to 0.015-8); Staphylococcus spp. = 0.12 (less than or equal to 0.015-1); Enterobacteriaceae = 0.03 (less than or equal to 0.015-1); Haemophilus influenzae = 0.015 (less than or equal to 0.004-0.03); Streptococcus faecalis = 2 (0.5-4). The MIC90 of ciprofloxacin for 98 tobramycin-resistant isolates was 2.0 micrograms/ml. Ciprofloxacin was stable at temperatures of 35, 22, 4, -20 and -70 degrees C for up to 15 weeks. Varying the inoculum from 10(3) to 10(7) colony forming units per ml or the pH from 6-9 resulted in no increase in the MIC. Of the new quinoline derivatives tested, ciprofloxacin demonstrated consistently superior antibacterial activity. Ciprofloxacin may be particularly effective for oral administration in infections requiring prolonged antibiotic therapy, such as bone, joint, and complicated soft tissue infections, and in pulmonary infections in patients with cystic fibrosis.