AIM: To determine the anti-inflammatory activity of probiotic Bifidobacteria in Bifidobacteria-fermented milk (BFM) which is effective against active ulcerative colitis (UC) and exacerbations of UC, and to explore the immunoregulatory mechanisms.METHODS: Peripheral blood mononuclear cells (PBMNC)from UC patients or HT-29 cells were co-cultured with heat-killed probiotic bacteria or culture supernatant of Bifidobacterium breve strain Yakult (BbrY) or Bifidobacterium bifidum strain Yakult (BbiY) to estimate the amount of IL-10 or IL-8 secreted.RESULTS: Both strains of probiotic Bifidobacteria contained in the BFM induced IL-10 production in PBMNC from UC patients, though BbrY was more effective than BbiY.Conditioned medium (CM) and DNA of both strains inhibited IL-8 secretion in HT-29 cells stimulated with TNF-α, whereas no such effect was observed with heatkilled bacteria.The inhibitory effect of CM derived from BbiY was greater than that of CM derived from BbrY.DNAs of the two strains had a comparable inhibitory activity against the secretion of IL-8.CM of BbiY induced a repression of IL-8 gene expression with a higher expression of IκB-ζ mRNA 4 h after culture of HT-29 cells compared to that in the absence of CM.CONCLUSION: Probiotic Bifidobacterium strains in BFM enhance IL-10 production in PBMNC and inhibit IL-8 secretion in intestinal epithelial cells, suggesting that BFM has anti-inflammatory effects against ulcerative colitis.