It is shown that when dilute rabbit serum rich in agglutinin for the hog cholera bacillus is heated at 75°C. for 20 minutes and the bacilli incubated with the heated serum, agglutination fails to result on the addition of unheated immune serum. When the immune serum is first heated to 80°C., it no longer greatly inhibits secondary agglutination when the organisms are exposed to fresh agglutinin. The abortion bacillus agglutinin acts in a similar manner except that the immune serum must be heated above 80°C. for 20 minutes to prevent the second agglutination. The reactions are specific since control experiments with normal rabbit serum heated at various temperatures failed to influence further agglutination. It has also been shown by precipitation tests that there is definite fixation of serum proteins and bacterial cells with the heated sera which would prevent subsequent agglutination. Furthermore, heated antiserum which would prevent the secondary agglutination still possessed the property of deviating complement in a hemolytic series.

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