Ciprofloxacin-d8 is intended for use as an internal standard for the quantification of ciprofloxacin by GC- or LC-MS. Ciprofloxacin is a fluoroquinolone antibiotic. It is active against a variety of Gram-positive and Gram-negative bacteria in vitro, including S. aureus, L. monocytogenes, P. aeruginosa, Legionella, N. gonorrhoeae, and H. pylori (MIC50s = 0.004-1 µg/ml). It is also active against clinical isolates of Bacteroides, Fusobacterium, Eubacterium, Actinomyces, Peptococcus, Peptostreptococcus, and Streptococcus in vitro (MIC50s = 0.5-2 µg/ml). Ciprofloxacin inhibits S. aureus DNA gyrase and topoisomerase IV (IC50s = 13.5 and 5.76 µg/ml, respectively). It reduces mortality in mouse models of intraperitoneal E. coli, P. vulgaris, K. pneumoniae, P. aeruginosa, and S. aureus infection (ED90-100s = 1-5, 2.5-5, 5-10, 20-40, and 80 mg/kg, respectively) and prevents mortality in a mouse model of subcutaneous S. typhimurium infection at 10 mg/kg. Formulations containing ciprofloxacin have been used in the treatment of bacterial infections.