The humoral response to the gastrointestinal (GI) flora was analyzed in secretory Ig (sIg)-deficient polymeric IgR (pIgR)(-/-) mice and otherwise congenic C57BL/6 mice. While both strains carried an ileal flora of similar size and composition, increased bacterial translocation to mesenteric lymph node was demonstrated in pIgR(-/-) mice. Serum IgA was greatly increased in pIgR(-/-) mice compared with C57BL/6 mice and reacted with commensal organisms and food. Serum IgG levels in pIgR(-/-) mice were increased to 6-fold above that of C57BL/6 mice and included specificities that bound to selected flora antigens. The enhanced recognition of flora antigens in pIgR(-/-) mice was explored using ovalbumin (OVA)-specific CD4(+) T cells and feeding of low concentrations of OVA. Increased proliferation of transgenic T cells was observed in pIgR(-/-) mice, relative to C57BL/6 mice, suggesting elevated net uptake of protein antigens from the GI tract in the absence of sIg. These studies suggest that there is increased recognition of GI flora antigens by systemic antibodies in pIgR(-/-) mice, most probably as a result of increased access of antigens from the GI flora to the systemic immune compartment, and support the hypothesis that a major function of the secretory immune system is to return environmental antigens to mucosal surfaces.