In addition to their antibacterial properties, quinolones are capable of modulating the immune response. The aim of this paper was to study the effects of a new fluoro-4-quinolone on the respiratory burst. We evaluated the effects of marbofloxacin on the activation of peripheral phagocytes in non-infected bovines after a 5-day treatment. The immunopharmalogical study measured the chemiluminescence response of phagocytic cells obtained from total blood. Serum cortisol and albumin levels were also measured. Data showed that the treatment with marbofloxacin induced a mild decrease in the oxidative response. There was no significant difference between serum albumin levels of normal values and those of before and after treatment, and the levels of serum cortisol were also not significantly different before and after treatment. These results suggest that marbofloxacin treatment could modulate the inflammatory response of phagocytic cells by counteracting the oxidative burst.