During the course of a study of P/eiffer baciUi, strains were grown on special transparent agar and it was observed that when one particular strain was grown in this way two kinds of colonies developed. The colonies of one kind were like those usually described as typical for this organism while the colonies of the other kind were opaque and were very iridescent when viewed by oblique transmitted light. It was also observed that the organisms forming the colonies of one kind differed in morphology from the organisms forming colonies of the other kind (1). Further study indicated that the phenomenon was undoubtedly an example of variation or bacterial dissociation. During recent years much new and important knowledge concerning variations in bacteria of other species has been obtained (2) and it has seemed important to study in greater detail, in the light of this new knowledge, the variations occurring among the so-called influenza bacilli, or Pfeiffer bacilli.The bacteria of this group do no t form a well characterized bacterial species, and it has long been recognized that individual strains differ from one another in morphology and virulence, in the appearance of the colonies which they form, in their ability to form indole, in power to ferment sugar and to induce hemolysis, in their immunological reactions, and even in their requirements for g~owth in artificial cultures. The literature relating to the biological characteristics of the bacteria forming this heterogenous group has been recently reviewed by Jordan

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The influence of living Bordetella pertussis on the induction and duration of pathophysiological reactions in mice infected intranasally with graded doses of culture was studied. Lethally infected mice showed loss of body weight, spleen atrophy, pronounced hypothermia and hypoglycemia, and highly elevated levels of leukocytes and serum immunoreactive insulin. Sublethally infected mice showed normal weight gain, practically normal temperature, spleen enlargement, lesser pronounced hypoglycemia, lower but significantly elevated levels of leukocytes and serum immunoreactive insulin, and histamine sensitization. Intensity of each reaction was related to the degree of lung infectivity. Hypothermia and leukocytosis were highly correlated. Concentration of serum immunoreactive insulin was closely related to the level of leukocytosis but not to the level of glucose. The strain and age of mice significantly affected the degree and duration of the reactions. The results suggest that the intranasally infected mouse may provide a useful model for investigations on whooping cough.