Immunofluorescent analysis of viral antigens in the cultured fibroblasts and renal tissues in patients with IgA nephropathy was described. Freeze and thawed extracts of pharyngeal cells obtained from patients with IgA nephropathy, chronic proliferative glomerulonephritis without IgA deposition (PGN) and healthy adults were cultured with human fibroblasts, i.e. Hel cells, with or without addition of 5-iodine 2'-deoxy-uridine (IUDR) at 37°C for 2 weeks. These fibroblasts and renal sections were stained with several kinds of FITC-labeled antiviral antibodies. Deposition of adeno, herpes simplex, varicella zoster or parainfluenza 3 was observed not only in the renal sections but also in the nuclear regions and/or cytoplasm of Hel cells after incubation of extracts of pharyngeal cells with or without IUDR from patients with IgA nephropathy. It is indicated that antigenic stimulation in the upper respiratory tracts may be due to several different types of DNA and/or RNA viruses in patients with IgA nephropathy. It appears that these antigenic substances show some heterogeneity among these patients.