Ever since it has been known that immune sera often agglutinate the bacteria concerned in their production, constant efforts have been made by investigators to determine the value of this reaction in the case of various bacteria, both as a means of diagnosis of specific diseases and as a method of differentiating bacteria which closely resemble each other. Streptococci have been included among the bacteria thus studied, and certain facts have been learned regarding the power of sera to cause their agglutination.