Subclinical mastitis is a disease of major economic importance to dairy industry causing reduced milk
quality and loss in its production. Therefore, the present study was carried out for isolation and
identification the main pathogen responsible for this disease and detection the drug of choice. A total of
475 milk samples of lactating dairy cows were collected from different localities in El-Sharkia
Governorate, and subjected to physical, chemical tests (California mastitis test (CMT) as well as Somatic
cell count (SCC), bacteriological examination and antimicrobial susceptibility pattern. The prevalence
of subclinical mastitis at cow level was 21% (100/475). Accurately 80 Staphylococcal isolates were
identified with prevalence of 61% S. aureus through beta hemolysis and coagulase positive. The
obtained S. aureus isolates were highly sensitive to vancomycin, gentamicin, ciprofloxacin and
chloramphenicol with percentages of 93.4%, 83.6%, 82% and 80.3%, respectively. Meanwhile, high
frequency of resistance was observed to oxacillin, erythromycin and tetracycline, with percentages of
59%, 55.7%, and 47.5%, respectively. Therefore, the main choice drugs for subclinical mastitis were
vancomycin and gentamicin.