Kefir, which is a fermented milk product that was recently introduced in Japan is becaming available for widespread consumption. Because there is the effect of controlling the growth of food poisoning bacteria in acid milk, it is expected that the growth of E. coli O157: H7 in kefir can be similarly controlled. When E. coli ATCC or E. coli O157: H7 was cultivated in a pH-neutral condition, the growth of the bacterium was not influenced at all regardless of the presence of kefir. On the other hand, when E. coil ATCC or E. coli O157: H7 was cultivated in pH-acid condition, the growth of these was markedly inhibited. The growth of E. coli ATCC of 10⁹ cfu/ml or E. coli O157: H7 of 10⁷ cfu/m l to which kefir of pH 4 was added was especially completely inhibited in a short time. These results suggested that an antibacterial substance to the growth of E. coli ATCC or E. coli O157: H7 was existed in kefir.