One important feature in the successful isolation of B. influenzæ from all cases that has been brought out in the course of the work here reported, is the necessity of making simultaneous cultures from all portions of the respiratory tract, since by no single culture method was it found possible to find the organism in all cases. It has been pointed out that one of the most characteristic local phenomena of the disease is the rapidly progressing attack upon the mucous membranes of the respiratory tract. It seems quite possible that B. influenzæ in predominant numbers at least may be found in many cases only at the crest of the wave, if we may speak of it as such. By way of analogy is the well-recognized fact that the successful isolation of streptococcus from cases of erysipelas often depends upon taking cultures from the margin of the advancing lesion. While definite proof is lacking for this opinion, it would seem to receive some support from the observation that B. influenzæ rapidly disappears from the throat with the onset of convalescence in a considerable proportion of cases. It is felt that these observations, establishing the predominance of B. influenzæ in the early acute stages of the disease, are of considerable significance, especially when exactly the reverse condition was found in studying the incidence of the organism in cases of measles. In consideration of the primary cause of influenza, attention has often been focused upon the many different bacteria found in autopsy cultures. The most prominent of these are the ill-defined diplostreptococci of the European writers, the various immunologic types of pneumococci, and S. hemolyticus. Other microorganisms less frequently found are staphylococci, M. catarrhalis, nonhemolytic streptococci, and B. mucosus capsulatus. It is not within the scope of this paper to discuss their relation to the various types of pneumonia found at autopsy, but their very multiplicity would seem sufficient _prima facie_ evidence that they bear no etiologic relationship to influenza and must be regarded only as secondary invaders. If any further support for this opinion were necessary, it may be found in the studies upon the incidence of pneumococcus and S. hemolyticus in early cases of influenza described in this report. Both were found to occur in the same proportions in which they may be found in normal individuals at any time. Although Pfeiffer maintained that B. influenzæ was found only in true epidemic influenza, the incorrectness of this contention has been thoroughly established by many reliable investigators and it has been shown beyond question that influenza bacilli may always be found in a small proportion of normal individuals and are not infrequently found in other respiratory diseases.