OBJECTIVES In vitro and experimental studies in animals have established the anti-inflammatory effects of moxifloxacin. Cardiopulmonary bypass (CPB) leads to an inflammatory response. The aim of this study was to assess whether the inflammatory cytokine response to CPB is reduced with a perioperative antibiotic prophylaxis, either moxifloxacin or cefuroxime (the standard prophylaxis). PATIENTS AND METHODS Twenty-eight patients scheduled for elective coronary artery bypass grafting with CPB were randomly assigned to receive either moxifloxacin or cefuroxime as the perioperative antibiotic prophylaxis. Interleukin (IL)-6, -8, -10 and tumour necrosis factor-α (TNF-α) serum concentrations were determined at eight time points before and after CPB. RESULTS In both groups, all cytokine concentrations significantly increased after the start of CPB. There were no statistically significant differences between the moxifloxacin and cefuroxime groups at any point; IL-6 concentrations [median (interquartile range)] 240 min after CPB, the primary endpoint, were 364 (192-598) and 465 (325-906) pg/mL (P = 0.323), respectively. CONCLUSIONS Neither moxifloxacin nor cefuroxime produced significant attenuation of the inflammatory cytokine response to CPB. The reasons why moxifloxacin did not have significant anti-inflammatory effects in this unique clinical situation may be: (i) the inflammatory response to CPB may be different from that of infectious disease states that were used to establish the immunomodulatory effects of moxifloxacin; and (ii) a single intravenous dose, which was used in this investigation, may not lead to high enough plasma and intracellular concentrations.