On the Precipitation reactions in homologous and heterogous antigen-antibody systems, the author studied the immuno-chemical properties of aspirin between anti-aspirin-proteins rabbit sera with homologous antigen (bovine serum globulin-aspirin, human serum aspirin) and heterogous antigen (rabbit serum globulin-aspirin) as aspirin-protein conjugates. And also the inhibitory action of aspirylglycine on the antigen-antibody reaction was studied and the following results were obtained. 1. It is possible to confirm in vitro serological antigen-antibody reaction between the aspirin-protein conjugates and the anti-aspirin-protein rabbit sera. 2. It is also possible to confirm the immuno-chemical properties of aspirin from the cross reaction occurring in the antigen-antibody reaction. 3. From this immuno-chemical characteristic it has been possible to recognize that aspirin as a hapten possesses antigenicity. 4. In the above-mentioned cross reaction, inhibitory phenomenon by aspirylglycine is established. 5. The inhibitory phenomenon by aspirylglycine can be recognized not only in the antigen titer but also in the antibody titer.