A study on the specificity of antibodies eluted from renal biopsy specimens from patients with Henoch-Schönlein purpura (HSP) nephritis, IgA nephropathy and other glomerular diseases is described. The specimens were treated with citrate buffer, pH 3.2, and the ‘eluate’ was neutralized by sodium hydroxide. The ‘eluate’ was then applied to the acid-treated sections obtained from the same patients and from other patients with HSP nephritis as well as to the sections from patients with IgA nephropathy and other glomerular diseases. The ‘sections were stained with FITC-labeled heavy-chain specific anti-human IgA antisera and then examined with a fluorescent microscope. It was demonstrated that the antibodies obtained from patients with HSP nephritis specifically recombined with the glomerular mesangial areas in patients with IgA nephropathy and vice versa, whereas they did not combine with renal tissues obtained from patients without IgA nephropathy. These antibodies did not show antimesangial activity since they did not react with normal glomeruli or nephritic glomeruli other than those from cases with HSP nephritis and/or IgA nephropathy. The specificity of IgA antibodies was confirmed by various tests using anti-human IgA antisera, human IgA myeloma protein, whole serum samples, human γ-globulin absorbed with anti-human IgG and/or IgM antisera or clostral IgA. It is suggested that some common antigens may be involved in the development of HSP nephritis and IgA nephropathy.