Viruses have been suspected to be etiological agents of IgA nephropathy. Recently, viruses were detected in renal tissues from patients with IgA nephropathy. We tried to cause lesions similar to IgA nephropathy by inoculating virus into mice and to detect virus RNA in the lesion by in situ hybridization. A group of mice were inoculated intravenously with coxsackie B4 virus once a month from 1 to 5 months of age and sacrificed monthly from 6 to 12 months of age. Mesangial proliferation and deposits that stained positive with periodic acid-Schiff in light microscopy and electron-dense deposits in electron microscopy were found from 6 months of age. Positive findings for IgG and IgA deposition in the mesangium were noted and the intensity of IgA deposition was predominant after 10 months of age. The signals of coxsackie B4 virus by in situ hybridization were observed in the lesions. These observations indicate that coxsackie B4 virus inoculated repeatedly into mice induces lesions similar to IgA nephropathy. The depositions of the lesions may be immune complexes of coxsackie B4 virus and these immune complexes injure renal tissues.