Acute inflammation and edema of the interlobular septa of the lung with no suppuration is often found with both lobar and bronchopneumonia and is occasionally so far advanced that it can be recognized on gross examination of the lungs. In a small area interlobular septa are conspicuous as yellowish lines of edematous appearance which may be 1 to 1.5 mm. in thickness and sometimes form a network with rectangular or polygonal meshes. The gelatinous appearance of the edematous fibrous tissue does not suggest suppuration. Microscopic examination shows that the tissue is distended by edema and contains fibrin and polynuclear leucocytes; the lymphatics are distended and contain a network of fibrin within which leucocytes are numerous. Inflammatory edema of the interstitial tissue has been recognized at autopsy four times in association with bronchopneumonia (Autopsy 253 with Pneumococcus II; Autopsy 335, with Pneumococcus IV and S. viridans; Autopsy 477 with S. hemolyticus and Autopsy 498 with S. viridans); twice with lobar pneumonia (Autopsy 343 with Pneumococcus IV and Autopsy 353 with atypical Pneumococcus II); twice with combined lobar and broncopneumonia (Autopsy 273 with S. hemolyticus and Pneumococcus IV and Autopsy 357 with Pneumococcus IV). Edema of interstitial septa was recognized at autopsy in the immediate neighborhood of an abscess three times (Autopsies 277 and 278 with hemolytic streptococci and Autopsy 282 with hemolytic streptococci and Pneumococcus II). In these instances of inflammation and edema the lymphatics are found distended by fibrinous thrombi, and it is probable that occlusion of lymphatics determines the occurrence of inflammatory edema within the surrounding tissue. Inflammation has not proceeded to suppuration. With interstitial suppurative pneumonia, interlobular connective tissue is marked by conspicuous yellow lines, 1 to 3 or even 5 mm. in thickness, forming a network with polygonal meshes which represent secondary lobules (Figs. 10 and 11). The distended septa not infrequently have bead-like enlargements at intervals and from the cut surface it is often possible to scrape away creamy yellow pus. These lines of suppuration invariably extend up to the pleura and are often broadest immediately below it. Adjacent septa which have not undergone suppuration are much thickened and have the yellowish gray appearance produced by edema. [Illustration: Fig. 10.—Interstitial suppurative pneumonia; interstitial septa are the site of suppuration and lymphatics are distended with purulent fluid; empyema. Autopsy 474, left lung. (See right lung) Fig. 9. ] [Illustration: