To test the hypothesis that IgA nephropathy can result from a mucosal immune response, mice were orally immunized with one of three protein antigens for 14 wk. Such mice exhibited an essentially pure mucosal antibody response characterized by specific IgA-producing plasma cells in exocrine sites and specific IgA antibodies in serum. Furthermore, 73% of immunized mice had IgA and 88% had immunogen deposited in the glomerular mesangium, and 64% of immunized mice examined ultrastructurally had electron-dense mesangial deposits. All three were present concurrently in 57% of the immunized mice. No differences in regard to IgG or IgM were observed between immunized and control mice for any of these parameters. Mucosal immunization therefore can result in a specific immune response that leads to mesangial deposition of immune complexes containing IgA antibody. In its fundamental features the experimental renal lesion resembles that seen in the human disease IgA nephropathy.