By means of complement fixation test the author conducted a comparative study on the reactivity of the serum of the rabbit immunized with bovine thymus nucleoprotein on different visceral extracts of bovine and swine, and obtained the following results. 1. Aqueous extract of bovine thymus nucleoprotein possesses antigenicity. 2. The grades of reactivity as observed in antigen-antibody dilution test about bovine thymus aqueous extract, bovine thymus nucleoprotein, swine thymus aqueous extract, and swine thymus nucleoprotein against the serum of the rabbit immunized with bovine thymus nucleoprotein are in the order of bovine thymus nncleoprotein=bovine thymns aqueous extract > swine thymus nucleoprotein > swine thymus aqueous extract. 3. Bovi...