Liquid chromatography-mass spectrometry (LC/MS) based label free quantitative proteomics analysis was applied for identification of differentially expressed proteins among the whey samples isolated from, 1) milk from cows with no history of mastitis, negative for S. aureus and somatic cell count (SCC) >2×105 cells\ml and 2) milk samples from sub-clinical mastitis., i.e., positive for S. aureus and SCC >2×105 cells\ml.