The early diagnosis of mastitis represents an essential factor for a prompt detection of the animal for further actions. In fact, if not culled, infected cows must be segregated from the milking herd and milked last, or milked with separate milking units. Besides microbiological analysis, the somatic cell count (SCC) commonly used as predictor of intramammary infection, frequently lead to a misclassification of milk samples. To overcome these limitations, more specific biomarkers are continuously evaluated. Total amino acid content increases significantly in mastitic milk compared to normal one. Bovine mastitis can arise as a result of infection of the mammary gland by Staphylococcus aureus. Multiplication of this bacterium within the mammary gland is required for infection to persist. S. aureus requires branched-chain amino acids (BCAAs: isoleucine, leucine, valine) for protein synthesis, branched-chain fatty acids synthesis and environmental adaptation by responding to their availability via transcriptional regulators. The importance of BCAAs for S. aureus physiology necessitates that it either synthesize them or scavenge them from the environment. Increase of BCAAs in composite milk has been postulated to be linked to mammary infection by S. aureus. In the present work, we demonstrated, by a direct ion-pairing reversed-phase method based on the use of the evaporative light-scattering detector (IP-RP-HPLC-ELSD), applied to 65 composite cow milk samples, a correlation between the concentration of isoleucine and leucine and S. aureus load.