Aims: To determine the role of lactoperoxidase (LP) in inhibiting the growth of microorganisms in pasteurised milk. Methods and Results: Four micro-organisms of importance in the spoilage of pasteurized milk were challenged in lactoperoxidase (LP)-enriched ultra-heat treated (UHT) milk after subsequent pasteurization. Milk samples were stored at the optimum temperatures for growth of the individual bacteria. Pasteurization was carried out at 72 degreesC/15 s and 80 degreesC/15 s to determine the effect of the LP system on the micro-organisms. An active LP system was found to greatly increase the keeping quality (KQ) of milks inoculated with Pseudomonas aeruginosa, Staphylococcus aureus and Streptococcus thermophilus and pasteurized at 72 degreesC, but had little or no effect in milks heated at 80 degreesC, presumably due to virtual inactivation of LP at 80 degreesC. However, pasteurization temperature had no effect on the KQ of milks challenged with Bacillus cereus spores. Conclusions: This study suggests that the LP system, rather than heat-shocking of spores, is responsible for the greater KQ of milk pasteurized at 72 degreesC/15 s compared with 80 degreesC/15 s. Significance and Impact of Study: The study emphasizes the care required in selecting pasteurization temperatures in commercial practice and to avoid the temptation to compensate for inferior quality of raw milk by increasing pasteurization temperature.