Abstract: Abstract: Plasmin(PL) is by far the predominant and most completely studied endogenous protease in bovine milk. PL activity level is an important factor affecting dairy product quality and its processing characteristics. Raw milk with a high PL level can have detrimental effects on the quality of pasteurized milk and UHT milk, causing undesirable flavor, precipitation and gelation, and shortening the shelf life of the product. Considerable research has focused on minimizing the activity of PL in fluid milk. Many factors, such as pH, mineral content, whey proteins, and storage temperature can influence the kinetics of the PL-induced hydrolysis. However, according to China's current raw milk production situatioin, the objective of this study is to determine how such factors such as cattle breed, parity, dairy farming mode, and milking time affect PL activity in milk; thermal stability of plasmin is also studied. The results show that the PL activity between dairy farming mode, parity, and cow breeds are significant (P<0.05). Although there are effects of dairy farming mode, parity, and cow breeds on plasmin activity, they are lower in large scale farming, parity (1,4), and Jersey cows. However, the effect of the milking time on PL activity is not significant (P>0.05). PL activity of raw milk gradually decreases with increasing thermal treatment temperature. However, the thermal treatment associated with pasteurization of milk (75℃, 15 sec) has been shown to increase PL levels. This increase in PL activity has long been attributed to inactivation of plasmin inhibitors(PI) and plasminogen activator inhibitors(PAI), but plasminogen activators (PA)is even more heat stable than PIand PAIduring pasteurization and UHT. In this temperature range, plasminogen (PG) loses its naturally occurring tertiary structure, but is not yet inhibited. On the contrary, PG becomes more accessible to the action of PA by unfolding of its kringles. Activation of PG is enhanced upon pasteurization heating. Further heat treatment has been shown to reduce PL levels. PL activity only decreases 25 percent after pasteurized (75℃, 15 sec) treatment. The effects of cysteine addition and heat treatment on PL activity is also studied. The results show that cysteine has some inhibited effect on PL activity in milk. When milk was stored after heat treatment (75℃), there is a decrease in PL activity. PL activity is significantly affected due to interactions with cysteine, which contains free SH groups that cause irreversible denaturation of PL by S-S/S-H interaction. When milk is heated to higher temperatures of 85℃ or 90℃, a larger decrease in PG activity and no further increase in PL activity is observed. This is due to PG interaction with cysteine. PG activity is tested in the presence of cysteine, which causes a marked decrease in PG activity. Two-dimensional electrophoresis confirms that heating of PG in the presence of cysteine causes polymerization of PG into dimers, trimers, and polymers through disulfide interchange. Formation of polymers hinders the activation of PG into PL. This result suggests that the addition of cysteine prior to the pasteurization significantly decreases the potential for proteolysis caused by the PL system during storage. This study has certain reference value to the quality evaluation and grading of raw milk.