Enrofloxacin-d5 is intended for use as an internal standard for the quantification of enrofloxacin by GC- or LC-MS. Enrofloxacin is a fluoroquinolone antibiotic. It is active against panels of Campylobacter, E. coli, and Salmonella isolates (mean MIC50s = 0.06, 0.5, and 0.03 µg/ml, respectively). It is also active against equine isolates of L. intracellularis (MICs = 0.125-0.5 µg/ml). Enrofloxacin inhibits bacterial DNA gyrase. In vivo, enrofloxacin (10 mg/kg) increases survival in a mouse model of enteropathogenic E. coli-induced sepsis. Formulations containing enrofloxacin have been used in the treatment of veterinary bacterial infections.