Fluorescein labelled IgG fractions of serum from patients with acute post-streptococcal glomerulonephritis (AGN) stained the glomerular basement membrane of renal tissue from the same and other patients with AGN obtained during the early phase of the disease. Using a spectrofluorometric method it was quantitatively shown that the staining capacity of these serum fractions was reduced after they had been preabsorbed with disrupted nephritogenic group A streptococci (type 12, 49 and a non-typeable strain) or their isolated plasma membranes. No such effect was observed when group A streptococci of types 3, 4, 6, 12, 14 and 25 or their plasma membranes obtained from patients without AGN were used for preabsorption. Plasma membranes of the nephritogenic type 12, 49 and the nontypeable strain diminished the staining capacity of the patients' labelled IgG fractions significantly, regardless of the serotype of the original nephritogenic streptococci isolated from the individual patient. These results indicate the presence of specific nephritogenic, type independent antigens in the plasma membrane of certain group A streptococci which appear to be shared by different serotypes.