Whole blood or sera were collected from individuals who had had infections with Group A streptococci of known serological type as long ago as 10 to 32 years. Most of these patients had not been treated with chemotherapeutic drugs. By means of bactericidal tests with all these sera, and mouse protection tests with some, type-specific antibodies could be demonstrated in at least half of them after a lapse of many years, the longest interval being 32 years.

Two biological methods for estimating the amount of M antigen produced by Group A streptococci are described. By selecting strains for use by these methods, optimal proportions of M antigen and antibody could be employed in the tests and small amounts of antibody were, therefore, demonstrable.

The assay methods for M antigen are also of value for other experimental purposes.

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