The term interstitial bronchopneumonia has been used by MacCallum to designate a lesion which he has found in association with measles at Fort Sam Houston. This name he states does not describe accurately the early stage of the lesion, for its interstitial character is not evident at first. In his monograph on “Epidemic Pneumonia in the Army Camp,” published in 1919, MacCallum describes and pictures instances of the lesion which we have designated interstitial suppurative pneumonia and classifies them as interstitial bronchopneumonia. We have shown that this lesion, which is the result of infection of the lymphatics with S. hemolyticus, bears no necessary relation to the lesion which is characterized in its early stage by peribronchiolar pneumonia and in its later stages by chronic inflammation with mononuclear infiltration and proliferation of the peribronchial, perivascular and interalveolar tissue. At Fort Sam Houston, nearly every patient with measles was infected with hemolytic streptococci; we observed, following influenza, similar prevalence of hemolytic streptococci in certain wards in the base hospital at Camp Pike. Among the cases at Fort Sam Houston there were doubtless instances both of interstitial suppurative pneumonia caused by hemolytic streptococcus and of chronic bronchopneumonia not referable to this microorganism. Studying pneumonia following influenza at Camp Lee, Va., and later at Camp Dix, N. J., during the fall of 1918, MacCallum reached the conclusion that “interstitial bronchopneumonia” following influenza was caused by B. influenzæ of Pfeiffer. This lesion attributed to B. influenzæ differed from that previously referred to hemolytic streptococcus in the following characters: the lymphatic channels in the bronchial walls and widened interlobular septa are inconspicuous and none are found distended with exudate; there is no intense infection of the pleura, and polynuclear leucocytes are inconspicuous in the alveolar exudate and in the walls of the bronchi. It seems probable these differences are explained by the absence of hemolytic streptococci which tend to invade lymphatics and produce severe inflammatory changes in the pleura.