Endostreptosin (ESS) is an antigen derived from the cytoplasm and the plasma membrane of nephritogenic group A and to a lesser extent group C and G streptococci. It is immunologically not related to streptococcal exoenzymes or the streptococcal cell wall and can be detected on the endothelial side of the glomerular basement membrane of kidney biopsies of patients with acute poststreptococcal glomerulonephritis (ASGN) during the early phase of the disease. Highest and most persistent antibody titers to this antigen are found in patients with ASGN. Immunoaffinity isolated ESS was injected i.v. into the tail vein of Wistar Furth (W/FU) rats for up to five successive days. The animals were sacrificed on days 1, 2, 3, 4, 5, 8, 9, 10, 11 and 12. Frozen sections of the rat kidneys were tested by immunofluorescence against rabbit anti-ESS-antibody and against sera from patients who had recently recovered from ASGN as well as against anti-rat-IgG and C3. The basement membranes of rat kidneys were positive for ESS deposition starting from day one on, but were negative for anti-rat IgG and C3 during the first four days. Rats sacrificed on days 8-12 showed increasing deposition of IgG and C3 with decreased staining for ESS. The sera of rats sacrificed on days 1-3 had no detectable anti-ESS antibodies; whereas animals from day 4 on had low levels of anti-ESS antibodies as determined by microcomplement fixation. Control animals showed no staining for ESS, IgG, C3 as well as no detectable anti-ESS antibodies.