Twenty-two strains having the typical morphological and cultural characteristics of B. pertussis have been studied by means of the agglutination and agglutinin absorption reactions. These tests have demonstrated that the cultures studied fall into two serological groups. If the two groups are tentatively designated as “a” and “b” the results may be briefly described as follows: Anti-serums for group “b” agglutinate the strains of group “b” but agglutinate the strains of group “a” very slightly or not at all. The absorption of group “b” serum by group “a” strains does not appreciably reduce the agglutinins for group “b.” Group “a” serum, however, agglutinates group “b” strains to a considerable extent. The absorption of group “a” serum by group “b” strains results in a reduction of the agglutinins for strain “a.” The serological differences, therefore, are sharply defined in one direction, but group relationship is shown in the reverse direction. These findings are of immediate interest because of their po...