We investigated the role of lipopolysaccharides (LPS) in glomerular IgA deposition of ddY mice. The incidence of IgA deposition was lower in the LPS-injected mice at 10 and 13 months of age as compared with that of their age-matched controls (10 months, LPS 20% vs control 60%; 13 months, LPS 14% vs control 100%). Serum levels of IgA were lower in the LPS-injected mice than in the control mice. There were no appreciable differences in the percentage value of Thy-1+ cells, the ratio of Lyt-1+/Lyt-2+ cells, mitogenic responses, or IL-1 activities between the LPS-injected and the control mice. On the other hand, the IgA responses of spleen and Peyer's patch cells in the LPS-injected mice were lower than those observed in the control mice. These results suggest that the persistent use of LPS directly suppresses gut-associated lymphoreticular tissue (GALT), and that the lower IgA response in GALT induced by LPS causes suppression of glomerular IgA deposition in ddY mice.