A method for detecting type-specific antibody to type 12 ß-hemolytic streptococci is described. The method is based on the utilization of a partially purified I131 labeled streptococcal acid extract and the solubility of this extract in 40 per cent saturated ammonium sulfate as compared to the insolubility of antibody and antigen-antibody complexes at this salt concentration.

The salient features of this technique are:

(a) When absorbed sera were used, no non-type-specific reaction with antigen occurred.

(b) The sensitivity of the system allowed detection of type-specific antibody in hyperimmune antisera that had been diluted 1200-fold.

(c) The method does not rely on secondary manifestations of antigen-antibody interaction for the detection of antibody.

(d) As little as 0.03 µg of M12 protein was detected when soluble unlabeled M protein was used to inhibit the reaction between I131 M12 protein and anti-M12 antibody.

(e) The kinetics of the reaction between M protein and anti-M antibody can be studied, thereby making information available as to the quality as well as the quantity of antibody produced in this antigen-antibody system.

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