The lactoperoxidase system is a naturally occurring antimicrobial system found in milk, with lactoperoxidase, thiocyanate and hydrogen peroxide as its components. The keeping quality of milk pasteurized at 72°C for 15 s was found to be better than that of milk heated at 80°C for 15 s. This agrees with previous findings and is usually attributed to heat shocking of spores. However, complete deactivation of lactoperoxidase occurred at 80°C–15 s, whereas at 72°C–15 s residual lactoperoxidase activity was ∼70%, which may provide an alternative explanation. Higher levels of hypothiocyanite (the major antimicrobial agent produced by the lactoperoxidase system) were also detected in milk processed at 72 than at 80°C, which supports the theory that the lactoperoxidase system has a role in the keeping quality of pasteurized milk. Of all the methods evaluated, titratable acidity and alcohol stability gave the most consistent estimates of keeping quality, while dissolved oxygen was a good indication of the onset of spoilage. Lactoperoxidase activity decreased with temperature more rapidly between 70 and 80°C than is usual for an enzyme over a 10 deg C range.