Plasmin (PL), a serine-proteinase, appears to be the predominant native proteinase in milk and it is mainly associated to casein micelles which represent its substrate (Bastian and Brown, 1996). Plasmin occurs in milk together with its inactive zymogene, plasminogen (PG) (Bastian et al. 1991). The cascade of reactions leading to plasminogen activation is regulated by a complex network of molecular interactions between plasminogen activators (PA) (tissue-type and urokinase-type) and at least three types of specific PA inhibitors (Politis, 1996). Stage of lactation affects PL and PA activities: late lactation is associated with higher activity of PL and PA (Baldi et al., 1996). Plasmin in milk is responsible for the hydrolysis of α- and β-caseins (Aslam and Hurley, 1997; Trujillo et al., 1997)...