Vertebrates have a large antibody repertoire with diverse antigen specificities, poised to react to invading pathogens, such as bacteria, viruses and helminths. In some species, microbes such as these are required for development of both diverse antibody and B-lymphocyte repertoires. Recent studies demonstrate that B-lymphocyte expansion, selection and somatic diversification of Ig genes are regulated in these species, in part, by the interaction between commensal intestinal bacteria and gut-associated lymphoid tissue (GALT). These findings indicate that the commensal microbiota can shape the repertoire of peripheral B lymphocytes and can potentially influence the health of the host. Here, how the interactions between commensal microbiota and lymphoid cells of GALT might affect the development of the peripheral B-lymphocyte repertoire is discussed.