Reports about the passive anaphylaxis using the yeast polysaccharide as the antigen are very few. After the Rosenau-Anderson's report, there appeared Axamit's one, but all of these are reports about the active anaphylaxis using the yeast cell and its crude extract. In regard to the passive anaphylaxis using the yeast polysaccharide as the antigen, only Kesten-Mott made a report after Tomcsik-Kurotchkin. So far as I know, there is no such report in our country, either. Though in these reports the minimal lethal antigen dose is referred to, the minimal lethal sensitizing dose is not referred to at all. The author made an experiment on the passive anaphylaxis using the yeast polysaccharide as the antigen in order to recognize its speciality, and obtained the knowledge of the ability of the sensitization by the anti-yeast rabbit serum and the knowledge of the decrease of the complement tiler, etc. Conclusions are as follows: 1) The passive anaphylaxis can be caused by using the yeast polysaccharide as the antigen. 2) The field of its reaction resembles in shape that of the antigen-antibody reaction in vitro at the antibody excess zone. 3) The minimal lethal sensitizing dose is 10 EH in all, in case of injecting the antigen which is suitable to the zone of antiserum used for the sensitization. 4) In case that the dose of the sensitization is 10 EH per 100 gms. of guinea pig's weight, the minimal lethal antigen dose is 1/16 z. 5) The decrease of the complement is not so evident in case of the passive anaphylaxis using a small amount of the antiserum which has the great ability for the sensitization. 6) In the experiment of the passive anaphylaxis, the sensitizing dose has more influence upon the decrease of the complement than the antigen dose.