From a review of the work of others and his own [ceter dot]experience, Jaffe questions the entity reticulo-endothelial system as a distinct and limited system, as formulated by Aschoff. In his opinion the capillary endothelium in general is capable of storing vital dyes without discrimination, provided the dye comes into sufficiently intimate contact with the cells. While ordinarily liver and spleen store most of the injected material under normal circulatory conditions, under altered pathological conditions, particularly those which change the distribution of the blood and the velocity of the circulation, cells other than reticulo-endothelial cells in the proper sense may remove foreign material from the blood stream. The author proposes to drop the term reticulo-endothelial system and to speak rather of active mesenchymatous tissue.