Serum protein fractions and total proteins were studied with bloods obtained form the rats exposed each to the partial-, whole-bodies and the transplanted tumors (Walker-256-carcinosarcoma transplanted in the right hind leg). The following results were obtained. 1. The electrophoretic variation induced in the sera of tumor-bearing rats (Group Ⅱ), and the content of total proteins decreased. Early irradiation to the tumor part of rats less induced the variations of the electrophoretic pattern and the decrease of the amount of serum total proteins. When the distant metastasis appeared during irradiation treatment, the electrophoretic patterns and content of total proteins changed proportionally to the variation in sera of Group Ⅱ. On the other hand, the γ-globulin fraction increased in the long-term survival rat. 2. The separation of the rat serum β-globulin into two peaks of β(1)-and β(2)-globulin was shown only in Group Ⅳ (late irradiation to the right hind leg). This finding supposed that some factors involve in the sera of rats with transplanted primary tumor grown up to a fixed size and guessed the appearance of the distant metastasis during X-irradiation. 3. The percentages of the albumin and γ-globulin decreased slightly and those of the α(1)-, α(2)and β-globulin increased slightly in the rats with 300 rad partial-body (the right hind leg) X irradiation daily for 20 days. 4. The remarkable decrease of the albumin and γ-globulin, the increase of the α(1) and β-globulin, the marked increase of the α(2)-globulin and the decrease of serum total proteins were demonstrated for the sera of rats with 1,000 rad whole-body X-irradiation at a time. These phenomena seem to be related to the destructive and reticuloendothelial injury by the exposure.