ABSTRACT
       Background and Objectives: Mastitis caused by Staphylococcus aureus is a widely distributed disease in cattle, goats and sheep. The infection is often subclinical in cattle, leading to reduced milk production and quality, but acute catarrhal or even gangrenous inflammation may also occur. The aim of this study was to investigate resistance of S. aureus isolates from bovine subclinical mastitis to a number of antibiotics.
          Methods: Milk samples were collected from 120 cows in different regions of Tabriz, Iran. Milk samples were cultured and bacteriological identification was performed. Antimicrobial susceptibility of the isolates was assessed by determining minimum inhibitory concentration. Plasmid DNA was extracted by an alkaline lysis method. 
         Results: The highest frequency of resistance was observed against gentamicin (100%) and β- lactam antibiotics including amoxicillin (96%), ampicillin (40%) and penicillin (96%). The isolates mostly contained large plasmids, which might harbor acquired antibiotic resistance.
         Conclusion: The results confirm the high frequency of antibiotic resistance among staphylococci isolated from bovine subclinical mastitis.
           Keywords: Anti-infective Agents, Cattle, Staphylococcus Aureus, Mastitis, Plasmids.