During bacterial bovine mastitis, the quality of milk is altered because of caseinolysis. Endogenous potential actors in milk responsible for this caseinolysis have been well studied, unlike the exogenous bacterial ones. The aim of this study was to evaluate the direct role in caseinolysis of a mammopathogenic strain, Escherichia coli P4. Secretion of at least 4 extracellular bacterial caseinolytic enzymes was highlighted by zymography, in 3 different growth media, and at each bacterial growth state, suggesting that their expression was constitutive. Different experimental conditions to evaluate caseinolytic potential did not show any significant caseinolytic activity of E. coli P4 and of the 4 extracellular proteases detected, suggesting that the high caseinolysis observed during E. coli bovine mastitis does result from endogenous milk actors.