IgG, IgA and IgM antibody activities in human serum to six dietary and eight gut-related microbial antigens were measured by an enzyme-linked immunosorbent assay (ELISA). IgG activities to five of the dietary antigens decreased with age; IgM activities to four of them were weaker in old people than in children. Old people showed weaker IgM but stronger IgG activities to some of the microbial antigens than children did. A decline in IgG and IgM antibody titres to most dietary antigens with increasing age is consistent with the development of systemic hyporesponsiveness due to continuous antigenic stimulation of the intestinal immune system. Persistence of microbial antigens in the gut, moreover, may lead to systemic hyporesponsiveness of IgM-producing cells. Concurrently raised IgG titres to three of the bacteria might be explained by antigenic stimulation outside the intestinal immune system.