Abstract: In this study, the effect of the Lactoperoxidase (LP) system on antimicrobial activity, aerobic count, alcohol test and lactoperoxidase activity were determined in raw cow milk collected locally in Chiang Mai, Thailand, during summer season (March-April 2006). The active LP system was found to greatly increase the Keeping Quality (KQ) of all milk samples (both untreated and Escherichia coli treated samples). In addition, in the presence of the LP system, the decrease in microbial contamination was observed at the rate of 87% (untreated milk sample, treatment B) and 78% (E. coli treated milk sample, treatment C), respectively. Present results clearly demonstrate that the LP system can serve as an alternative method to control the microbial growth in cow milk.