The study was conducted to explore LD50 and pathological changes in mice caused by enterohemorrhagic Escherichia coli O157∶H7.Methods weaned mice were inoculated via enterocoelia inoculation with different doses of E.coli O157∶H7 021210 strain and simultaneously treated with nalidixic acid to determine LD50.Many pathological changes were seen in mice caused by E.coli O157∶H7.