In the early postoperative period (third postoperative day) the colonic enterotomies show 45 per cent higher bursting pressure following administration of Aprotinin than the control group (p less than 0,02). On the fifth postoperative day no difference was noted between both groups. The interpretation of the results is difficult because specific parameters such as collagen content and collagenase activity were not determinated. The relationship between colonic anastomotic breakdown and collagenase and therefore the question of collagenase inhibition have to be discussed. It is suggested that activation of procollagenase is prevented because trypsin and kallikrein are inhibited by Aprotinin.