The absence of cellular elements in the alveolar exudate is frequently observed in influenzal pneumonia (Figs.XXI,XXII,XXIII). This picture has been reproduced experimentally in animals which have been rendered aplastic with benzol, especially with reference to their myeloid elements (160). Pneumonia produced by intratracheal insufflation is more rapidly fatal in aplastic animals, and it is conceivable that the absence of cellular reaction is an explanation for the lack of resistance demonstrated by the high mortality of influenzal pneumonia. Frequently the fibrinoserous mass scattered diffusely throughout the lung is rich in bacteria. In the absence of cells of the polymorphonuclear series, the bacterial development seems to be unrestricted. The aplastic exudate is associatedclinically with an absence of a myeloid reaction in the peripheral circulation. The leucocytic count may be definitely decreased, even though the tissues have been invaded by pyogenic organisms to which the usual response is a definite leucocytosis. The only explanation is that the myeloid structures have been injured, probably by the unknown virus of the disease.