With the purpose to study the antigenicity of bovine thymus and thymus nucleoprotein the author investigated the passive anaphylaxis in guinea pigs induced by the sera of rabbits immunized with bovine thymus aqueous extract or with bovine thymus nucleoprotein as well as passive Arthus phenomenon of guinea pigs elicited by evans blue method, and obtained the following results. 1. As for the passive anaphylaxis in guinea pigs by the serum of the rabbit immunized with bovine thymus aqueous extract, the bovine thymus and bovine thymus nucleoprotein clearly bring about anaphylaxis. 2. The same can be said in the case of passive anaphylaxis in guinea pigs by the serum of the rabbit immunized with bovine thymus nucleoprotein. 3. In passive Arthus phenomenon elicited by the serum of the rabbit immunized by bovine thymus aqueous extract, bovine thymus and thymus nucleoprotein elicit a strong reaction. 4. This is true even in passive Arthus phenomenon by the serum of the rabbit immunized with bovine thymus nucleoprotein. 5. There is no marked difference between the antigenicity of thymus aqueous extrct and that of thymus physiological salinesolution extract.