To determine whether the size of the intestinal bifidobacterial population can influence the immune response to poliovirus vaccination in infants, we set up a randomized, placebo-controlled trial. From birth to 4 mo, infants were given a fermented infant formula (FIF) or a standard formula (placebo). Bifidobacteria were quantified monthly in infant stools. Antipoliovirus IgA response to Pentacoq was assessed before and 1 mo after the second vaccine injection. Thirty infants were randomized, and 20 completed the study (nine in the placebo group and 11 in the FIF group). Fecal bifidobacterial level was significantly higher with the FIF group at 4 mo of age (p=0.0498). Furthermore, B. longum/B. infantis carriage was higher at 4 mo in the FIF group (p=0.0399). Antipoliovirus IgA titers increased after Pentacoq challenge (p <0.001), and the rise was significantly higher in the FIF group (p <0.02). Antibody titers correlated with bifidobacteria, especially with B. longum/B. infantis and B. breve levels (p <0.002). Infants who harbored B. longum/B. infantis also exhibited higher levels of antipoliovirus IgAs (p <0.002). In conclusion, the present results indicate that antipoliovirus response can be triggered with a fermented formula that is able to favor intestinal bifidobacteria. Whether this effect on the immune system is achieved through the bifidogenic effect of the formula (mainly through B. longum/B. infantis and B. breve stimulation) or directly linked to compounds (i.e. peptides) produced by milk fermentation remains to be investigated.