The bactericidal effect of acidic electrolyzed water (AEW) generated by an apparatus of EIKEN Industry Co, was investigated. We used 3 strains ofEscherichia coli O157: H7, 96003 strain producing VT1, 96140 and 96141 with both VT1 and VT2 which were isolated in Gunma Prefecture. The bactericidal effect of the AEW was investigated in a serial dilution of AEW with or without the presence of serum. No. 96140 was killed at 25% of AEW and No. 96003 and 96141 were killed at 10% of AEW within 1 min. The bactericidal effect of AEW was greatly decreased in the presence of a low concentration of serum as low as 1%.