Endogenous proteolysis in quarter milk fraction was studied after the infusion of 10 μg of lipopolysaccharide of E. coli, 64 h post-infusion, during a single milking. Several parameters were considered and compared to theuninfused heterolateral quarter: somatic cell count (SCC), proteose peptone content and plasmin activity. Qualitative change of proteose peptone fraction and somatic cell proteolytic activity were investigated. In milk of control quarter no increase of SCC was noticed, plasmin activity was the main origin of proteose peptone content. A very wide increase of SCC was measured in infused quarter between the 3 fractions of milk. In the same time, proteose peptone content increased and a specific peptide appeared in milk of infused quarter. Despite of the increase of SCC during milking of infused quarter, no qualitative change of cellular proteolytic activity was measured.