Intestinal mucositis is a frequent side effect in cancer patients who are treated with chemotherapy. There are no effective treatment strategies to date. To find a novel way to alleviate mucositis, the effects of selenium-enriched Bifidobacterium longum (Se-B. longum) in preventing irinotecan (CPT-11)-induced intestinal mucositis in a mouse model were investigated. We tested the ability of Se-B. longum (Se 0.6 mg/kg, 5x10(8) cfu/mice) to reduce small intestinal mucositis induced by CPT-11 (75 mg/kg, daily) injected intraperitoneally for four consecutive days in mice. Se-B. longum significantly decreased mortality induced by CPT-11 from 71.4% to 16.7%. CPT-11 induced body weight loss, which was alleviated by preventative and simultaneous administration of Se-B. longum. Se-B. longum significantly decreased the severity of diarrhoea from 11 to 4% compared to the CPT-11 group. Inflammation, including intestinal shortening and upregulation of tumour necrosis factor-a and interleukin-1 beta induced by CPT-11, were prevented by Se-B. longum. Se-B. longum is effective in preventing small intestinal mucositis induced by CPT-11 and therefore has potential to be used clinically by cancer patients.