It is now generally accepted that acute post-streptococcal glomerulonephritis (PSGN) is the consequence of the formation of antigen-antibody-complement complexes on the basement membrane of the glomerulus and that the antigen is of streptococcal origin. In cases of acute PSGN a high titre of specific antibodies to a streptococcal cytoplasmic extract can be found at the very beginning of the disease. This cytoplasmic antigen which we called endostreptosin (ESS) is probably the pathogenetic antigen of glomerulonephritis. It is deposited on the subendothelial side of the basement membrane in the first few days of the disease and is rapidly covered by newly-formed and specific antibody and complement with resultant immune injury causing signs and symptoms of symptomatic but also frequently asymptomatic acute glomerulonephritis. To further characterize and isolate ESS we used immunoaffinity chromatography and Western blotting techniques. PAGE analysis of the affinity-isolated ESS revealed the major component to have a molecular weight of approximately 45 kD. Sera from patients with PSGN or sera of rabbits immunized with affinity-isolated ESS reacted by Western blotting with at least one antigenic component with a molecular weight of approximately 45 kD. Normal human sera or the sera of non-immunized rabbits failed to demonstrate activity against this antigen. The basement membranes of the glomeruli of patients with very early PSGN stain with fluorescein-labelled gammaglobulin of patients with glomerulonephritis. This staining can be prevented when these sera are pre-absorbed with ESS but not by pre-absorption with intact cells or cytoplasmic extracts of other bacteria.