We examined whether adherence of enterohemorrhagic Escherichia coli (EHEC) O157:H7 to intestinal epithelial cells contributed to the induction of secretory immunoglobulin A (IgA) antibody production in mice. Wild-type EHEC O157:H7 and its mutants deficient in the espA , sepL , tir and eae genes, encoding adherent factors on the locus of enterocyte effacement (LEE), were inoculated intragastrically into mice. Inoculation of wild-type EHEC induced fecal IgA antibodies specific to EHEC at 4 weeks after the inoculation, but that of espA - and sepL -deletion mutants did not. Furthermore, even 4 inoculations at weekly intervals with espA -deletion mutant, heat-killed wild-type EHEC and nonpathogenic E . coli did not induce fecal IgA antibodies, although these bacterial inoculations induced serum antibodies. Kanamycin (Km)-treated mice showed prolonged and similar fecal shedding of Km-resistant mutants of EHEC O157:H7 including A2-F6 having intact LEE, A6-E7 ( sepL -insertion mutant), G1-E11 ( tir -insertion mutant) and Δ eae ( eae -deletion mutant). In this case, A2-F6 induced fecal IgA antibodies, but the other mutants with defective LEE did not. In contrast to the fecal IgA antibodies, serum IgM and IgG antibodies were induced in mice inoculated with any of the LEE defective mutants as well as A2-F6. Thus, adhesion of EHEC to epithelial cells is essential for inducing the mucosal immune response in the intestine but not for inducing the systemic immune response.