In a previous article (Ferry and Noble, 1918) we have described the cultural, agglutination and absorption reactions between Bact. pertussis and B. bronchisepticus and have shown that, while the two organisms are distinct, they are apparently somewhat closely related. The most striking characteristics of the organisms, according to the serological reactions, were shown to be the ability of B. bronchisepticus to produce an immune serum that would agglutinate both the B. bronchisepticus and Bact. pertussis anitigens and the ability of Bact. pertussis to produce an immune serum that would agglutinate only the homologous antigen. The absorption reaction showed that the B. bronchisepticus antigen would absorb from the antibronchisepticus serum (a serum that contained agglutinins for both organisms) only the B. bronchisepticus agglutinin (the major agglutinin) leaving intact the agglutinin for Bact. pertussis (the minor agglutinin). This minor agglutinin could only be absorbed by the Bact. pertussis antigen. This type of an agglutinin was termed by the authors a "transitive" agglutinin.The present investigation was undertaken to confirm the work of the previous paper through complement fixation tests and to determine, if possible, the value of this test in differentiating between the two organisms.Strains used. At first a large number of strains of each organism were used, -the same strains as those worked with in the pre-