Experimental mastitis induced by the S. aureus in six dairy cows was performed to investigate the relationship between somatic cell count (SCC) and proteolysis during early and chronic phase of infection (up to 28 days post-infection). An elevated level of infection was detected during the early phase and was characterized by a measurable link between SCC and proteose-peptone content. In contrast to this observation, no significant relationship was detectable between the two parameters during the chronic phase of infection. During this phase the measurement of SCC could not be established as a reliable indicator of milk proteolysis. Moreover, the results of this study demonstrated that the high risk of milk proteolysis associated with an elevated SCC content does not consistently obey the opinion that excessive SCC results in a high level of proteolysis of milk. Our results were compared with experimental E. coli mastitis in order to evaluate the differences that exist between the courses of bovine intramammary infection caused by S. aureus and E. coli.