In acute streptococcal infections, anti-streptolysin O (ASO) is produced in response to streptolysin O antigens liberated by the bacteria. Information on extent and degree of infection can be obtained by measuring serum ASO levels. Elevated ASO levels have also been found in patients suffering from scarlet fever, acute rheumatoid arthritis, tonsillitis, and other streptococcal infections as well as in healthy carriers