In retrospect we have found that 38 of 187 patients who fulfilled the criteria of mesangial IgA-nephropathy had possible acute glomerulonephritis at the onset of their disease. We have therefore studied anti-streptococcal antibodies (ASO and ADNAseB) prospectively. Forty-three per cent of the patients had ADNAseB greater than 800 units. Thirty-one per cent of the patients studied more than once had a fourfold or greater change in their ADNAseB titre. Thirty-three per cent of the patients had different groups of beta-haemolytic streptococci isolated from their throats. This indicates a possible role of beta-haemolytic streptococci in the pathogenesis of some cases of mesangial IgA-nephropathy.