For use as a mucosal adjuvant for human vaccines, a simple method has been developed for the affinity purification of recombinant cholera toxin B subunit which had been expressed in a safe host, Bacillus brevis. Recombinant cholera toxin B subunit, adsorbed quantitatively to a d-galactose-agarose column, was eluted with an 0.1–0.4 M d-galactose gradient with a yield of >90%. The cholera toxin B subunit preparation was similar to the native cholera toxin B subunit with respect to GM1 binding ability, remarkable stability of the pentamer, and the dissociation-reassociation property by shifting pHs. Cross-linking experiments with glutaraldehyde demonstrated that the pentameric form was predominant; tetrameric, trimeric, dimeric and monomeric forms were detected to a lesser extent, and additionally 10- and 15-mers were observed depending on the concentration of the cholera toxin B subunit.