Background: Bifidobacterium breve M-16V (M16V) is a probiotic bacterial strain with a long tradition of use in neonatal intensive care units in some countries. Previous study showed that the effects of M16V administration on gene expression were greater during the weaning period than in the neonatal period and were greater in the colon than in the small intestine and spleen, suggesting that M16V has anti-inflammatory effects. In this study, we evaluated the effects of inflammation during the weaning period and the effects of M16V on normal and inflammatory conditions. Methods: From postnatal day (PD) 21 to 34, weanling rats were administered of 2.5 × 10 9 of M16V daily, and colitis was induced by administration of 2% dextran sulfate sodium from PD28 to 35. Colitis severity, immune function, and microbiota were investigated. results: Colitis caused a reduction in body weight gain, colon shortening, poor nutritional status, anemia, changes in blood and spleen lymphocyte populations, spleen T-cell malfunctions, and alterations in colon microbiota. M16V administration improved some but not all of the changes induced by colitis. conclusion: M16V could suppress inflammation and, therefore, can be considered a safe strain to use not only during the neonatal period but also the weaning period.F ollowing delivery, mammalian neonates come out from a germ-free uterus to a highly contaminated extrauterine environment, in which they are exposed to many bacteria and antigens (1). During this process, trillions of microbes come to inhabit the mammalian neonatal intestine, and a complex symbiosis system termed microbiota is formed (1). Microbiota is considered important for normal immunological development of the host. For example, reports have shown reduced diversity in the intestinal microbiota and an association of reduced beneficial microbes, such as lactobacilli and bifidobacteria, with increased risk of allergic disease (2). Generally, bifidobacteria, a beneficial microbe, are more dominant in the intestinal microbiota of breastfed infants than in formula-fed infants (1,3). Additionally, the immune function of breastfed infants is reportedly different from that of formula-fed infants (4), and as a result, breastfeeding is thought to protect infants from diseases such as allergy and pneumonia (4). Consequently, numerous trials testing the benefits of probiotics against neonatal immunological development have been conducted (5). Lactobacilli and bifidobacteria are commonly used as probiotics; however, bifidobacteria are reported to be more predominant than lactobacilli in human neonatal intestines (3). There are many species within the bifidobacteria genus, among which, Bifidobacterium breve (B. breve) is one of the most abundant in human neonates (3).B. breve M-16V (M16V) is a probiotic strain that was isolated from the fecal sample of a healthy baby (6). This strain has been used for low-birth-weight infants in neonatal intensive care units in Japan for more than 20 y and proves to be a suitable probiotic strain for routi...