An in vitro investigation on sensitivity of some pathogens isolated from bovine, equine and swine species to chemotherapeutic agents diffusely used in veterinary medicine was carried out. In particular, it was evaluated the activity of benzylpenicillin, ampicillin, cefalexin, cefoperazone, ceftiofur, oxytetracycline, chlortetracycline, doxicycline, aminosidine, kanamycin, tiamphenicol, nalidixic acid, flumequine and enrofloxacin. Ceftiofur and enrofloxacin showed a greater antimicrobial efficacy; on the contrary, tetracyclines showed a smaller activity, whereas drugs diffusely used as aminoglycosides, tiamphenicol and β-lactams resulted poorly active.