Based on the experimental result in rabbit that the phagocytic activity of R.E.S. was enhanced conspicously by prolonged and large does of estradiol benzoate, further experiment was attempted to elucidate the effect of surgical removal of both ovaries, the main estrogen-producing organ in the body, on the phagocytic activity of R.E.S. organs, with the following results. In the oophorectomized rabbit, the disappearance of blocking agent injected into the ear vein was delayed, compared to that of normal rabbith, with coincidental decrease of phagocytic activity of liver, spleen and bone marrow. It can be assumed that the phagocytic activity of R.E.S. showed considerable decrease with oophorectomy.