Mouse pups born to and nursed by dams fed Bifidobacterium breve YIT4064 and immunized orally with rotavirus were more strongly protected against rotavirus-induced diarrhea than those born to and nursed by dams immunized with rotavirus only. The level of antirotavirus IgA in milk of dams fed B. breve YIT4064 and immunized orally with rotavirus was significantly higher than that of dams immunized with rotavirus only. The antirotavirus IgA level in feces of dams immunized orally with rotavirus was also increased by oral administration of B. breve YIT4064. These findings demonstrate that oral administration of B. breve YIT4064 enhanced antigen-specific IgA antibody in the mammary gland and in the intestine. Oral administration of B. breve YIT4064 may enhance antigen-specific IgA antibody against various pathogenic antigens taken orally and induce protection against infections in various mucosal tissues.