A study was conducted to determine whether intraperitoneal and oral administration of formalin-fixed gram-negative bacteria induced immunohistologically and ultrastructurally evident glomerular deposition of IgA and C3 in C3H/HeN mice. Separate treatments with strains of Pseudomonas aeruginosa, Escherichia coli, Haemophilus influenzae, Klebsiella pneumoniae, and two kinds of lipopolysaccharide (LPS) were used. Two mice in each treatment group were sacrificed at 10, 20 and 30 weeks of age to examine sequential glomerular changes. In addition to the intraperitoneal administration (IP) groups receiving whole formalin-fixed bacterial cells, cell precipitate and supernatant fractions of each bacterial strain after sonication were injected intraperitoneally once a week, and the mice were sacrificed at 30 weeks of age. Sequential quantitation or IgG, IgA or IgM in serum and the isotypes specific for each of the bacterial strains or LPS administered was performed by ELISA. The incidence of immunofluorescence positivity for glomerular IgA and C3 was 37-71 and 37-66.7%, respectively, in the IP groups that had received bacterial cells of each strain, which was significantly higher than that in the IP groups given LPS or in the controls. These results suggest that cell wall components common among gram-negative bacteria, other than LPS, play a major role in the glomerular deposition of IgA and C3. This is the first use of gram-negative bacteria to establish an active model of IgA nephropathy.