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. Bovine thymus nucleoprotein was fractionated into nucleic acid and protein (histone) and the reactivity of the histone sulfate as the antigen was tested against the serun of the rabbit immunized with the bovine thymus nucleoprotein. It showes almost the same reactivity as that of nucleoprotein and thymus aqueous extract. No antigenicity of nucleic acid can be recognized. 4. In the study of the reactivities of 21 different visceral extracts in bovine and swine against the serum of the rabbit immunized with bovine thymus nucloprotein by means of the antigen dilution test, the reactivity are great in the following order in both bovine and swine: thymus extract, thymus nucleoprotein spleen, extract, testis extract, kidney extract and cerebellum extract.