Milk obtained from cows which were either infected by clinical mastitis or had been subjected to intramammary infusion of Escherichia coli endotoxin possessed high counts of somatic cells and very high levels of proteinase activity which hydrolysed the caseins almost completely in a few hours at 37 °C. The rate of hydrolysis of β-casein was slightly greater than that of αs1-casein, but in both cases hydrolysis was enhanced by 6 cycles of freezing and thawing to disrupt somatic cell membranes. A study of the relationship between proteinase activity and cell count suggested that only some of the proteinase activity originated in the somatic cells and also that the identity of the cells making up the total cellular population was important. Maximum proteolysis occurred at 50–60 °C, but the temperature-activity curve was a broad peak. Likewise the pH versus activity plot was very broad and was almost flat over the pH range 6–9. Experiments with a number of inhibitors of proteinases failed to give a clear cut pattern of inhibition. All evidence obtained was consistent with the view that several different enzymes with different pH and temperature optima and different specificities contributed to the overall hydrolysis of caseins in these milks. From electrophoretic band patterns one of these enzymes was clearly plasmin, but in high cell count milks other proteinases also became significant.