The indigenous milk proteolytic enzyme, plasmin, is of technical importance as its hydrolytic action on casein may affect the quality of dairy products. The objective of this study was to examine the factors that affect plasmin-induced proteolysis during storage of milk and how such proteolysis may affect some properties of the casein micelles. Two competing mechanisms affected plasmin activity during storage at different temperatures, i.e., autolysis (self-hydrolysis) and plasminogen activation; at 5 degrees C, the former mechanism was more significant than the latter, while at 37 degrees C, activation predominated during the early stages of storage. At the latter temperature, however, similar to 65% of plasmin activity was lost after 10 d, due to autolysis. Changing plasmin activity influenced the rate of hydrolysis of beta-casein; at 37 degrees C, hydrolysis of beta-casein ceased when similar to 40% of the original beta-casein remained. In addition, the temperature of storage significantly affected the composition of the casein micelles. Dissociation of beta-casein during storage at 5 degrees C was significantly increased by preheating of milk (90 degrees C for 10 min); at 37 degrees C, dissociated beta-casein in the milk serum was readily hydrolysed by plasmin. While pre-heating milk to 90 degrees C for 10 min greatly reduced plasmin activity and proteolysis, addition of exogenous plasmin to heated milk appeared to promote the release of beta-lactoglobulin/kappa-casein complexes into milk serum. Overall, storage conditions had a considerable effect on the plasmin system and its subsequent action on casein micelles. (C) 2004 Elsevier Ltd. All rights reserved.