An extracellular exopolymer endowed with immunomodulating properties is prepared by culturing the Bifidobacterium infantis longum strain deposited at the Institut Pasteur under No. I-885 in a suitable medium under anaerobic conditions. After the microorganisms are discarded from the culture medium, the exopolymer thus produced is isolated, concentrated and then precipitated by adding an organic solvent; finally the product resulting from the precipitation is lyophilized.The exopolymer thus obtained possesses an activity on cellular as well as on humoral immunity, and could thus be used as an immunomodulating agent in any potential therapeutic application.