With an improved technique of complement fixation test the author studied the reactivity of the aqueous extracts of various bovine and swine viscera on the serum of the rabbit immunized by bovine thymus aqueous extract, and obtained the following results. 1. Grades of the reactivity of bovine thymus aqueous extract, bovine thymus nucleoprotein, swine aqueous extract, and swine thymus nucleoprotein are in the order of bovine thymus aqueous extract bovine thymus nucleoprotein > swine thymus aqueous extract=swine thymus nucleoprotein. 2. When the animal is immunized with bovine thymus aqueous extract, antybody against bovine thymus nucleoprotein is produced. 3. In the stndy of antigenicity of 21 different visceral proteins in bovine and swine by antigen dilution test, those that show a strong reactivity are thymus, thymus nucleoprotein, bone marrow, cerebrum, cerebellum, stomach, lung, spleen, thyroid, testis, epididymus and kidney in bovine, while they are thymus and cerebellum in swine.