The glomerular staining of a commercial polyclonal antiserum against cytomegalovirus (CMV) and of a mixture of 13 monoclonal anti-CMV antibodies in twelve patients with immunoglobulin-A (IgA) nephropathy was examined. The polyclonal antiserum stained the mesangium in all patients. The CMV staining coincided only partially with that of IgA in five patients and the staining distributions were discordant in six. The monoclonal antibodies stained mesangium in only one patient. Serum levels of IgA and IgG anti-CMV antibodies did not differ between patients and controls. In western blot analysis, the polyclonal antiserum reacted with three proteins in uninfected human fibroblasts. Absorption of the antiserum with uninfected fibroblasts reduced the reactivity on western blot analysis and eliminated glomerular staining in five of six renal biopsy samples. It is concluded that the previously reported binding of anti-CMV antibodies to glomeruli from patients with IgA nephropathy may be the result of contaminating antibodies to human antigens.