[en] The administration of antibiotics by aerosol to horses suffering from respiratory infections may partially circumvent the limitations of antimicrobial therapy, e.g. large injection volumes, low bioavailability and risk of diarrhea. Only injectable formulations are available currently and usually contain other substances that could irritate the mucosa and induce coughing and bronchospasm. In addition, the quality of the aerosol, particularly in terms of the delivery of antibiotics to the deep parts of the lung, is unknown. Although used under field conditions, cefquinome delivered by aerosol has never been studied in horses. This study examined the safety of cefquinome injectable solution, administered by aerosol at a dose of 225 mg/inhalation to 7 healthy horses, by assessing (1) pulmonary function before and 15 min after a single inhalation, at the first day (Day 1) and the fifth day (Day 5) of a 5 day period treatment; and (2) the inflammatory status of the lung, i.e. percentage neutrophils and myeloperoxidase concentration, based on bronchoalveolar lavage (BAL) at D1 and D5. In addition, cefquinome concentrations were measured in bronchoalveolar lavage fluid after aerosol, intravenous (i.v.) and intramuscular (i.m.) administrations. A single aerosol of cefquinome injectable solution did not induce any immediate nor delayed pulmonary side effects in healthy horses and produced cefquinome concentrations in bronchoalveolar lavage (BAL) within 30 min that were higher than the minimal inhibitory concentration of the main equine respiratory pathogens. These results should stimulate further studies, especially in horses suffering from bronchial hyper-reactivity. Aerosol delivery of antibiotics may well have a role in equine therapeutics.