After intranasal inoculation of 108.0 E.I.D. or more of CAM virus into mice, the acid precipitable material, Xerosin, from culture filtrates of Achromobacter Xerosis, n.sp. limited the development of pneumonia but did not alter the formation of non-infectious viral particles. Xerosin, however, did not diminish the lethal effects of Newcastle disease virus infection of chick embryos, nor did it inhibit the severe damage inflicted by this agent upon Ehrlich ascites tumor cells. In the mouse lung Xerosin suppressed lesions produced by chemical irritants in a manner similar to its inhibitory action upon viral pneumonia in mice. The action of Xerosin was shown not to be the result of a cortisone-like activity. Histologic studies indicated that Xerosin did not act to prevent damage of the specific host cells of influenza viruses, namely, the epithelial cells lining bronchi and bronchioles, but rather inhibited secondary reactions which lead to edema, hemorrhage and cellular infiltration manifested as pneumonia.