Mastitis is recognized as one of the most important diseases affecting the dairy industry. The antibiotic susceptibility test is important to achieve accurate treatment in subclinical mastitis. This study was conducted to determine the presence of contagious mastitis agents (Streptococcus agalactiae and Staphylococcus aureus) in 270 bovine milk samples collected from 132 dairy cows with subclinical mastitis in 15 different dairy farms located in the Marmara Region of Turkey. A total of 256 bacterial pathogens were isolated, of which 25 isolates were identified as S. agalactiae and 12 isolates were identified as S. aureus. Antibiotic susceptibilities of the isolates were investigated by disk diffusion method. All of the S. aureus isolates were resistant to ceftiofur and all of the S. agalactiae isolates were resistant to streptomycin. Sensitivity to other antibiotics tested was varied. The antibiotic susceptibility tests should be performed together with the identification of the bacterial agents in order to achieve effective treatment.