In the initial phase of acute poststreptococcal glomerulonephritis (AGN), antigenic sites not covered by specific antibody can be demonstrated in the glomeruli. These sites are on the endothelial side of the glomerular basement membrane and in the mesangial matrix. The antigen is contained in a water-soluble fraction of nephritogenic streptococci. It can be highly purified by column chromatography or acrylamide electrophoresis. This antigen against which the patient develops specific antibodies is contained and immunologically identical in all nephriotogenic streptococci examined so far irrespective of their M or T protein. It is not identical with any known streptococcal extracellular product. When antibodies are produced in rabbits against this streptococcal antigen they localize on kidney biopsies, obtained early during the disease from patients with AGN in an identical way as does the labelled antibody of the serum of patients wiht AGN. This antigen may be suited as a vaccine for the prevention of AGN.