In the rabbits, dogs and patients with parathion poisoning, the function of the reticulo-endotherial system was examined by means of carbon black phagocytosis (Sugiyama) and congo-red test (Adler & Reimann). Results are as follows: 1) In the acute parathion poisoning, the function of the reticulo-endotherial sytem presented itself the disturbances such as the decrease of phagocytosis and the increase of congo-red index at the early stage of the poisoning. 2) With daily injection of small doses of parathion, similar disturbances ever noted, and lasted longer. 3) The recovery of the function of the reticulo-endotherial system was similar to that of blood cholinesterase activity.