The _microscopical_ examination of the lung tissue confirmed the observations which were made in the gross. In the early stages of congestion the reaction was much more extensive than what could be spoken of as a broncho-pneumonia. The capillary dilatation in the alveolar walls occupied diffuse areas varying from multiple lobules and areas several cm. in size to the common diffuse congestion of an entire lobe. Capillaries were distended to their full capacity and often this engorgement was associated with the leakage of blood or a serous fluid. Not uncommonly a clear serous fluid was exuded into the interstitial tissues of the alveolar wall and collected within the air sacs. The high albuminous content of this fluid was seen in the homogeneous coagulation which occurred when the tissues were placed in fixatives. The microscopical sections of such parts demonstrated the coagulum occupying the alveoli as a clear homogeneous substance containing relatively few cells and looking not unlike the colloid deposit of the thyroid. The alveolar walls, themselves, were infiltrated with fluid so that the distended tissues and vessels made these structures thick and bulky. In our own observations we were impressed by the differences of the early inflammatory reaction from those ordinarily seen in pneumonia. Amongst these differences was the quantity of fluid extruded into the lung with a relative absence of fibrin. In some instances fibrin was completely wanting, although small quantities could be demonstrated in isolated areas. This observation upon the quantity of fibrin can be made only during the early stage of the disease in as much as after secondary infection of various kinds has become implanted the presence of fibrin has become a variable quantity often exceeding that seen in the early stages. This is one of the points upon which the older authors have laid stress in differentiating influenza pneumonia from others. In this we fully concur. Whether this lack of fibrin in the inflammatory exudate is a characteristic to be associated with the infection by the B. influenzæ alone is hard to say, but in as much as it was such a prominent finding we are led to lay some stress upon it. It is, of course, to be realized, as with all other micro-organisms that under certain conditions fibrin will form an important part of the exudate even when the B. influenzæ is present. This is true in the inflammatory reactions of the meninges present in infections due to this bacillus. Under the conditions of epidemic influenza where the lung lesion is the prominent and unique reaction this micro-organism fails by itself to bring out this quality in the exudate.