Evidence confirming the identity of fibrinogen-precipitating factor and streptococcal M protein is provided by the demonstration of bactericidal, mouse protective, and long chain-producing antibodies in the sera of rabbits immunized with washed M-fibrinogen precipitates. Two precipitin lines were observed in immunoelectrophoresis of human plasma vs. rabbit anti-M-fibrinogen antiserum; no precipitin reactions were observed between human serum and rabbit anti-M-fibrinogen antiserum. The reaction between M protein and fibrinogen was stoichiometric; a constant equimolar ratio of reactants was observed in precipitates formed with a wide range of M protein concentrations.

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