The inhibitory activity of fifty Bifidobacteria toward the infectivity of a rotavirus, which is the predominant cause of sporadic diarrhea in infants and young children, was investigated, and Bifidobacterium breve K-110 was found to have the most potent inhibitory activity. Accordingly, the rotavirus infection-inhibitory protein was purified, and its molecular weight was determined to be 76 kDa by SDS-PAGE. It was heat-labile and its 50% inhibitory concentration ($IC_{50}$) was 0.045 $mu g/ml$.