CI-934 is a totally synthetic difluorinated quinolinecarboxylic acid with an ethyl-amino-methyl pyrrolidine side chain, which has broad-spectrum antibacterial activity, including particular potency directed against streptococci and staphylococci. The CI-934 MIC (micrograms per milliliter) for 90% of the strains tested was 0.4 (range, 0.2 to 0.8) for a group of streptococci (pneumococci, viridans streptococci, Streptococcus faecalis, and Lancefield groups A, B, and C), 0.2 (0.05 to 0.2) for staphylococci (including methicillin-resistant Staphylococcus aureus), 0.025 (less than or equal to 0.003 to 0.025) for Haemophilus influenzae and Neisseria gonorrhoeae, 1.6 (0.1 to 25) for Enterobacteriaceae, 25 (3.1 to 25) for Pseudomonas aeruginosa, and 1.6 (0.05 to 3.1) for non-Bacteroides anaerobe species. CI-934 was equally active in vitro against multi-drug-resistant and -sensitive isolates, and cross-resistance was not apparent. Potency increased with alkalinity and was somewhat lower in urine. CI-934 was bactericidal. Inhibitory activity was generally unaffected by anaerobiosis, light, changes in inoculum size or cation concentration, or addition of human serum or sodium cholate.