The effects of the cell viability and integrity of Bifidobacterium on suppression of allergy were investigated. C3H/HeJ mice were sensitized on weeks 3, 4, 6, and 8 with ovalbumin and choleratoxin to induce an allergic reaction. Mice fed 0.2% of live, disrupted, or heat-killed Bifidobacterium bifidum BGN4 in the pellets of their diet for 8 weeks starting 2 weeks before initial sensitization differentially suppressed the allergy response in terms of levels of IgE and IgG1 in their sera, and symptoms on their tails. Viable Bifidobacterium was more effective than disrupted or heat-killed cells in suppressing the allergy. Growth inhibition, which occurred in the sham group at week 4, did not occur in the treated groups. These results show that Bifidobacterium has a suppressive effect on the allergic response of mice, and that the viability and integrity of the Bifidobacterium is required for effective suppression in our experimental model.