The author refined the yeast polysaccharide from the dry yeast by means of Hiokas's method, and experimented on the variation of its antigen activity in the refining process, the relation between its concentration in the blood and that in the urine, and its placental permeability, employing a guinea pig given the refined yeast polysaccharide by the abdominal cavity injection or the intravenous one.The anti-yeast rabit serum, (the antigen titer by Uhlenhuth's Method being 1 : 2,500,000), was used for proving the yeast polysaccharide in the blood, the urine and the amniotic fluid, and Uhienhuth's Method was employed for the measuring. The author could clarify the following points: 1) In case of refining the polysaccharide from the yeast, the degree of the antigen activity in the last process is 100 times as much as that of the extract by the physiological saline. 2) The yeast polysaccharide given to a guinea pig by the injection is rapidly excreted into the urine. 3) The yeast polysaccharide in the blood has shown the highest concentration for 4 hours since the injection, and thereafter it begins to decrease rapidly, to be found no more in the blood on the 6th day. 4) The yeast polysaccharide in the urine has shown the highest concentration since the injection, and thereafter it decreases a little less rapidly than that in the blood, to be found no more in the urine on 6th or the 7th day. 5) There is the proportional relation between concentration of the yeast polysaccharide in the blood and that in the urine. 6) In case of giving the yeast polysaccharide to a pregnant guinea pig by the injection, the concentration of the yeast polysaccharide in the embryo's serum, which is trans mitted from its mother, is 1/10-1/20 of that of the mother. 7) The yeast polysaccharide is never transmitted into the amniotic fluid. 8) The length of pregnancy has no relation to the degree of the yeast polysaccharide transmission to embryos.