The results of the present study revealed that peptides derived from bovine casein hydrolysates can promote peroxidase-dependent oxidation of human low-density lipoproteins (LDL). The reaction was independent of the free metal ions but required casein-derived peptides with tryosyl-residues, implying that the tyrosyl radical is a diffusible catalyst that conveys oxidizing potential from the active site of the heme enzyme to LDL lipids. This mechanism is independent of the peroxidase used to oxidize tyrosyl residues since myeloperoxidase and horseradish peroxidase mediate a similar LDL peroxidating process. Vitamin E, ascorbic acid, butylated hydroxytoluene and reduced glutathione delayed LDL oxidation and were consumed during the reaction, they transfered hydrogen to repair tyrosine.