A study was undertaken on the specificity of circulating IgA antibodies in patients with IgA nephropathy detected by immunofluorescence using avidin-biotin complexes. Renal biopsy specimens and serum samples were obtained from 33 patients with IgA nephropathy, 14 other glomerular diseases and 3 normal renal tissues. These renal specimens were treated with citrate buffer (pH 3.2), and then incubated with serum samples obtained from the same and other patients with IgA nephropathy, other glomerular diseases or healthy adults at 37 degrees C for 30 min. The specimens were incubated with biotin conjugated gout F(ab')2 anti-human IgA antiserum at 37 degrees C for 30 min, and then with fluorescein-labeled avidin at 37 degrees C for 30 min. It was found that IgA antibodies in the sera from patients with IgA nephropathy specifically combined with the autologous glomerular mesangial areas, but only 25.7% of them combined with allogeneic renal tissues of IgA nephropathy patients. Confirmatory findings were obtained using an automatic image analyzer. However, these IgA antibodies did not combine with the renal tissues from patients with other glomerular diseases or normal renal tissues. In parallel studies, in order to distinguish IgA nephropathy from other glomerular diseases before renal biopsy, the renal specimens from patients with IgA nephropathy were also incubated with serum samples obtained from 42 patients with proteinuria and/or hematuria before renal biopsy. It was demonstrated that the incidence of IgA binding in IgA nephropathy was significantly higher than that in other glomerular diseases prior to renal biopsy.(ABSTRACT TRUNCATED AT 250 WORDS)