1. A study was made on the fine structure and function of the sinus endothelial cells of lymph node to know more precisely about the features of them with reference to the relation to the reticuloendothelial system. 2. In normal condition they showed no phagocytic activity whereas reticulum cells in the sinus showed marked phagocytic activity. 3. Electron microscopically they were identical with the proper endothelia of blood vessels. Namely they had a basement membrane and connected with each other by tight junction structures. 4. These results led to the conclusion that the sinus endothelial cells of lymph node had to be omitted from the categories of reticuloendothelial system. 5. However, they turned to phagocytic after the foreign body stimulation for 2 weeks. 6. This phenomenon showed a possibility that the reticuloendothelial cells were not specific cells but only the proper endothelia which have been situated in an irritative circumstance. 7. The significance of these results on the concept of reticuloendothelial system was briefly discussed
