1. Experimental cerebral and meningeal tubercles in the rabbit are formed from cells of endothelial origin.
2. These cells are derived apparently from other sources than the neighboring capillary endothelium alone.
3. The circulating macrophages, which, in this case, are capable of multiplying by mitosis while still free in the blood, are drawn upon in the formation of the cerebral tubercle.
4. Splenectomy has not materially decreased the available supply of circulating macrophages in this experiment.
5. While these cells may originate in the endothelium of the liver and bone marrow, the lung appears to play a much more important rôle in this respect than has been hitherto suspected.