Kefir which is acidifying and alcoholic fermented milk is accepted as a good example of a probiotic mixture of bacteria and yeast. In this study; we planned to investigate the antimicrobial effect of 24 and 48 hours fermented kefir against Staphylococcus aureus (ATCC 29213), Bacillus cereus (ATCC 11778), Salmonella enteritidis (ATCC 13076), Listeria monocytogenes (ATCC 7644) and Escherichia coli (ATCC 8739) by using disk diffusion method. The best antimicrobial effect of 24 and 48 hours kefir was seen against Staphylococcus aureus with the diameter zones of 21.4 and 21.1 mm respectively at the end of fermentation period. Similar results were obtained from 24 and 48 hours fermented kefir on the 1st, 4th and 7th days of storage at + 4 ºC. In all cases, although the antimicrobial activity decreased or did not change during the storage; it increased only against Salmonella enteretidis.