The link between oxidation and increased proteolysis in raw milk was studied. To accelerate oxidation, H2O2 (1 mM) was added to raw milk, resulting in enhanced proteolysis by up to 11.2% after 24 h incubation at 5 degrees C. Addition of Cu2+ (10 microM) to milk or exposure of milk to light (60 min) likewise increased proteolysis. To explain the mechanism responsible for increased proteolysis as a result of oxidation, the effect of lipid oxidation products on plasmin-induced proteolysis was tested. Addition of malondialdehyde to skim milk increased the formation of gamma-caseins, a proteolysis product from plasmin hydrolysis of beta-casein. The same observation was made in a model system containing 4.5 g beta-casein/l sodium tetraborate buffer at pH 8 and plasmin. Addition of a plasmin inhibitor blocked the formation of gamma-casein. The results indicate that aldehydes accumulated from lipid oxidation can modify beta-casein and thereby increase susceptibility of the proteins to proteolysis. Furthermore, the data suggest that proteolysis in raw milk may be connected to oxidative processes.