Indirect semi-skimmed ultra high temperature (UHT) milk was made from refrigerated milk preserved by the lactoperoxidase system (LPS). The effect of the LPS on the physicochemical and biochemical properties of UHT milk during storage period of 6 months at 30°C was assessed. The levels of soluble nitrogen at pH 4.6 and non protein nitrogen in all UHT milk samples increased during storage. However, control UHT milk, manufactured with refrigerated milk which LPS was not activated; showed the highest values (p < 0.01). Less hydrophobic amino acids concentrations, such as found in nitrogen soluble fraction, were lower in activated LPS UHT milk. Also, bacterial proteinase activities, measured using ophthaldialdehyde (oPA) as substrate, increased during storage. They were found higher in control UHT milk than in treated milk (p < 0.01). However, a decrease of total fatty acids in UHT milk, dependent on time of storage and LPS activation, was found during UHT milk storage.