A study on the specificity of IgA in the supernatant of Epstein-Barr Virus (EBV) transformed lymphocytes obtained from patients with IgA nephropathy is described. Renal biopsy specimens were obtained from nine patients with IgA nephropathy and nine patients with other glomerular diseases. Mononuclear cells obtained from two patients with IgA nephropathy and two healthy adults were transformed with EBV and the supernatant of such transformed cell culture was applied to acid (pH 3.2)-treated renal sections obtained from the same and other patients with IgA nephropathy, as well as to those with other glomerular diseases. The sections were stained with FITC-labeled heavy chain-specific anti-human IgA antiserum and then examined with a fluorescent microscope. It was demonstrated that IgA in the supernatant specifically bound with the glomerular mesangial areas in patients with IgA nephropathy, while it did not bind with such areas in patients with other glomerular diseases. Although IgA in such samples bound with autologous renal tissues, about 50% bound with allogeneic renal specimens of IgA nephropathy. IgA in the supernatant of transformed lymphocytes from healthy adults was not bound with the glomerular mesangial areas of patients with IgA nephropathy and other glomerular diseases. It is concluded that IgA in the supernatant of lymphocytes by EB virus transformation is specific to the mesangial areas of IgA nephropathy and that some heterogeneity is seen among patients with this disease.