Abstract: The antibacterial effect of mixed culture of Lactococcus lactis subsp. lactis and Lactobacillus delbrueckii subsp. bulgaricus against some food borne pathogenic organisms were investigated. Much amount of acidity was produced by lactic acid bacteria and increased gradually with the fermentation time. Lating of reaching pathogens to milk lead to the over increasing of acidity produced by lactic acid bacteria. Over production of acidity aod reduction of pH level and other substances not determined lead to prevent or inhibit the growth of all pathogens. These factors clearly showed the inhibition effect of lactic acid bacteria against pathogens. The count of all pathogenic, organisms decreased sharply with fermentation time until it disappeared completely after 3 days. This shows the importance of the consumption of fermented milks. Cell-free extract of lactic acid bacteria shows clearly an inhibitory activity against all pathogenic organisms used. But, non-boiled extract was highly active than those of boiled ones. This means that, lactic acid bacteria produced thermolabile substances inhibit or prevent the growth of pathogens. Finally all these observations show the hygienic and nutritional importance of the consumption of fermented milks.