Within the alveolar tissue of the lung, spaces are seen lined by a layer of fibrin which stains homogeneously and very brightly with eosin. They are recognized as alveolar ducts by the presence of scattered bundles of smooth muscle in their wall. The layer of hyaline fibrin overlying the surface of the alveolar duct usually forms a continuous lining and covers over the orifices of the alveoli which surround the alveolar duct. These ducts are rendered still more conspicuous by the character of their contents which exhibits a sharp contrast with that of the surrounding alveoli. The alveoli duct occasionally contains a bubble of air, but more frequently it is filled with serum in which red blood corpuscles are sometimes numerous. There is within the lumen scant fibrin and very few cells, among which polynuclear leucocytes are predominant. In the surrounding alveoli on the contrary leucocytes and fibrin are abundant. A similar change is found in the infundibula very clearly defined by their conical form, which is especially well outlined below the pleura or in contact with interlobular septa. The infundibulum is outlined by hyaline fibrin which passes over the orifices of the tributary alveoli and separates the serous contents of the infundibulum from the cellular fibrinous contents of the alveoli about. The lesion which has been described is often associated with acute bronchitis and bronchiolitis, and the alveoli immediately about the respiratory bronchioles may be filled with polynuclear leucocytes. It is very common to find large bubbles of air sharply defined within the purulent contents of the bronchiole. In some lobules the alveolar ducts, infundibula and alveoli intervening between these foci of leucocytic pneumonia are almost uniformly filled with fibrin and polynuclear leucocytes, but in other places the formation of complete layers of hyaline fibrin is in process. Bubbles of air are often seen within the alveolar ducts, and about them is an irregular layer of fibrin formed by the penetration of air into a channel previously filled with a loose network of fibrin containing serum in its meshes. The fibrin compressed against the walls of alveolar duct and infundibulum remains as a compact layer separating these structures from the alveoli which project from their walls. The bubble of air is doubtless later absorbed and replaced by serum, so that many alveolar ducts are filled with serum almost wholly free from cells, whereas alveoli outside the fibrinous membrane contain a network of fibrin with leucocytes in greater or less abundance. In association with this fibrinous pneumonia, which has been described, hyaline thrombosis of the capillaries is not uncommon. This hyalin material within the capillaries gives reactions of fibrin, and in sections stained by the Gram-Weigert method for demonstration of fibrin, these thrombosed vessels have the appearance of capillaries irregularly injected with a blue material.