Research on the biological activity of proteins has developed on the basis of studies carried on the chemical derivatives of proteins produced after a selective reaction between some chemical reagents and specific amino acid residual group in proteins. Following the techniques of G. C. Butler, C. R. Harington and M. E. Yuill (1940) the author produced aspirin-protein by chemically combining amino group of serum protein (ε-Amino group of lysine residual group) with aspirin, and carried out serological studies with this aspirin-protein, especially on the homogeneity between aspirin-protein and serum protein, and on the change in the property of proteins by the introduction of aspirin, and obtained the following results. 1. The free amino acid group consumed in the chemical combination of aspirin with protein can be confirmed by the ninhydrin reaction with reagent hydrindantin. 2. The viscosity of aspirin-protein is higher than that of normal serum protein. 3. And aspirin-protein has moved its isoelectric point towards the acidic side. 4. The aspirin group of aspirin-protein is possible to isolate and extract with toluene, after hydrolyzis aspirin-protein, and to be determined colorimeterically by Gerhardt's iron perchloride reaction.