Plasmin (PLM; E.C. 3.4.4.14) is the major proteolytic enzyme normally present in bovine milk. From a technological point of view, PLM activity in milk is detrimental as it increases the proteolysis of casein to proteose peptones, and this results in reduced storage time, taste defects, loss of cheese yield and quality, and changes in the physicochemical properties of milk (for review, see Fox, 1992). Therefore a reduction of PLM activity in milk would be desirable. Plasminogen (PLG), the zymogen of plasmin, and PLM content are affected by several physiological and environmental factors, and by genetic factors such as breed (Richardson, 1983; Schaar, 1985; Politis et al. 1989; Benslimane et al. 1990). We have addressed the question of PLG polymorphism in dairy cattle.