1-Cyclopropyl-6-fluoro-1-dihydro-4-oxo-7-(1-piperazinyl)-1,4-dihydro-3- quinolinecarboxylic acid (ciprofloxacin, Bay-o 9867) a new quinoline carboxylic acid derivative, was tested in vitro against 233 various Gram-negative microorganisms, mostly resistant to nalidixic acid. Minimal inhibitory concentrations (MIC) against the Enterobacteriaceae and P. aeruginosa ranged between less than 0.003 to 1 mg/l and less than 0.003 to 8 mg/l respectively with MIC90 of 2 mg/l. However, when sensitivities were repeated with an acid broth they were increased by greater than or equal to 32 fold. This effect was more prominent when as nutrient pooled human urine was used and particularly for P. aeruginosa strains. Ciprofloxacin at a dose of 250 mg, or 500 mg, 12-hourly for 10 days was randomly given to 40 patients aged 23-76 years, suffering from upper (27) and lower (13) urinary tract infections (UTI) as proved from the "antibody coated bacteria" (ACB) test. Pathogens included E. coli (20), Proteus sp. (13), K. pneumoniae (1), C. freundii (1) and P. aeruginosa (5), with MICs between less than 0.06 to 2 mg/l. During treatment all but one of the patients responded favorably both clinically and bacteriologically, while at a six-week follow-up, nine patients with upper UTIs and underlying chronic pyelonephritis or/and structural abnormalities had relapsed, while only one became reinfected. Treatment schedule did not influence the results. No appreciable side effect or toxicity was observed. It is concluded that ciprofloxacin should have an important role to play in the treatment of UTI as well as in systemic infections whenever multiresistant pathogens are implicated.