The antigenic specificity of the mesangial IgA in IgA nephropathy (IgAN) remains unknown. Because shared antigenic specificities may be reflected in the usage of shared idiotypes, we prepared five monoclonal anti-idiotypic antibodies (MoAbs) specific for the mesangial IgA eluted from the kidney of an IgAN patient. All five MoAbs reacted with the same idiotype, which proved to be of a public nature. Although the idiotype could be identified in the mesangial deposits of the majority of IgAN patients studied, it was not specific for the disease because it was also found in the glomerular deposits of other types of glomerulonephritis. The idiotype was also expressed in polyethylene glycol precipitates of sera and in pokeweed mitogen-induced plasma cells from both IgAN patients and healthy controls. The conclusion that no disease-specific idiotypes are present in the renal eluate was further supported by the failure to produce polyclonal anti-idiotypic antibodies by immunizing a rabbit with the eluted mesangial IgA. Our results support the concept that mesangial IgA deposits in IgAN are of a polyclonal nature.