The present report compares the extracellular proteins of streptococci by sodium dodecyl sulfate polyacrylamide electrophoresis. A marked variation in the streptococcal extracellualr proteins (SEP) of different strains was detected, even in strains of similar serotypes. It was possible, however, to identify a single protein band that occurred predominantly in the SEP of strains isolated from patients with acute poststreptococcal glomerulonephritis (APSGN). This protein was generally not produced by streptococci obtained from patients without this disease. It appears to be immunologically similar in the various serotypes of streptococci isolated from patients with APSGN and can be demonstrated by immunofluorescence techniques to be present in the glomeruli of these patients.