The purulent sputum obtained during life and the exudate at autopsy from the later stages of lobar pneumonia commonly erode the surface of Löoffler's blood serum. Cellular material obtained from the pneumonic lung in an early stage of lobar pneumonia failed to erode the surface until washed with normal saline solution. Mixtures of washed pneumonic cellular material and normal human serum fail to erode Löffler's blood serum when the amount of cellular material is less than one part of cells to approximately three parts of serum. Erosion occurs when the cellular material exceeds this amount in the ratio.