Rate constants of enzyme proteins with hydroxyl radicals were estimated by summation of rate constants of amino acids toward hydroxyl radicals on the assumption that the radical reacts only with amino acid residues located on surface of an enzyme molecule. Kinetic basis of the assumption was discussed in terms of reaction in solution. The calculated rate constants for lysozyme, cytochrome c, ribonuclease, and α-chymotrypsin were found to be consistent with the experimentally obtained ones.

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Three lactating goats were administered with a single dose of 1 mCi of ²⁰³Hg intravenously. The ²⁰³Hg concentration in the blood and milk, and its excretion rate in urine and feces, were investigated. The ²⁰³Hg concentration in the whole blood was 5.5 × 10^-3 % of dose/g 1 hr after administration and decreased with time to 5.0 × 10^-5 % of dose/g 21 days after administration. On the other hand, the concentration in the milk reached a peak value of 5.7 × 10^-4 % of dose/g 5 hrs after administration, followed by a gradual decrease to 5.0 × 10^-6 % of dose/g 21 days after administration. The ²⁰³Hg concentration in the blood serum was higher than that in the red blood cells until the initial 5 hrs after administration. This relation was reversed thereafter. The ²⁰³Hg concentration in the milk casein was higher than that in the whey. The milk casein showed a peak value of 1.2 × 10^-3% of dose/g at 5 hrs after administration. The cumulative amounts of ²⁰³Hg discharged in the milk, and in the urine and feces for 21 days after administration, were about 0.9, 12 and 30 % of dose, respectively. The biological half life of ²⁰³Hg in these goats was estimated to be 26 days.

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¹³⁷Cs level in the total diet of Japanese reached the highest peak (107 pCi/day/person) in 1964, and then decreased year by year. The ¹³⁷Cs level in 1971 was less than 10% of the maximum value in 1964. Daily intake of stable cesium through the total diet has been in the range of 3.9 μg and 25.1 μg. The specific activities (¹³⁷Cs pCi/mg of stable Cs) of total diet were found to vary among districts. Average ratio of Cs atom to one million atoms of potassium in the diet was 1.7±0.7. Contribution of nine foodstuffs to the total intake of ¹³⁷Cs was calculated, and that of cereals was the highest and amounted to approximately 40% of the total ¹³⁷Cs intake. Using (¹³⁷Cs)-K observed ratio (OR_{human body-total diet}) for the Japanese adults of 2.35, the cumulative internal radiation dose to the whole body due to ¹³⁷Cs was estimated to be 7.3 mrads during the 9 years period from 1963 to 1971.

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The dose-response relationship of chromosome aberrations in the peripheral blood lymphocytes was studied in blood samples obtained from the patients who were exposed to a single dose of ⁶⁰Co γ-rays for their malignant diseases. The level of chromosome aberration yield, as studied 24 hours after irradiation, was not simply related to the radiation dose per se, but body weight, irradiated volume and radiation-induced inactivation of lymphocytes were found to be other important factors intimately related to the frequencies of chromosome aberrations; aberration yield expressed on the simple per-cell basis was roughly proportional to the irradiated volume and inversely proportional to the body weight.
The intercomparison of the frequency of chromosome aberrations in these locally irradiated persons with that obtained by the in vitro experiments suggested the D₃₇ value of about 350 rads for the survival of lymphocytes as measured 24 hours after irradiation.

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Ovariotomy in 8 week-old, specific pathogen free ICR/JCL mice was effective in securing a marked thymus regeneration following a single whole body irradiation with 300 rads of X-ray. Recovery of the bone marrow cell counts, especially of the lymphoid cells, preceded the thymus regeneration and was more marked in spayed mice than in the sham-operated. On the basis of these results, it is suggested that ovarian hormones play a role in the thymic lymphopoiesis by affecting the dynamics of the lymphoid stem cells in the bone marrow and/or the lymphopoietic microenvironments in the thymic cortex.

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Mice were exposed to 800 R whole body X-irradiation at a rate of 15 R per minute. Following exposure there was an increase in the permeability of L-alpha hydroxy acid oxidase from mouse liver peroxisomes, and an increased sensitivity of the peroxisomes to low concentrations of Triton X-100, These results indicate that latency characteristics of peroxisomes from irradiated animals are altered compared to the latency characteristics of peroxisomes from control animals.

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Effects of drying, a frequently used storage condition, on various physicochemical characteristics of DNA has been examined in DNA solution prepared from dried-DNA and original, native DNA. Drying resulted in a decrease of viscosity, a decrease of flow birefringence, a decreased sedimentation in neutral sucrose gradient, and little change in melting temperature of DNA. They imply that the drying caused a reduction of molecular weight and molecular size by 10-20 percent with little alteration in double strand character.
Gamm-irradiation of DNA solution prepared from native and dried DNA showed a remarkable difference in their responses to radiation. Thirty krads of gamma-rays caused dried-DNA to decrease its molecular weight by 70 percent and its melting temperature by 26°C, while it caused native-DNA to decrease its molecular weight by 10 percent and its temperature by 10°C. The apparent efficiency of double-strand scission of dried-DNA was 20 eV per one scission, and that of native DNA was 200 eV. The increased radiosensitivity of dried DNA might be resulted from formation of ‘hidden damages’ through drying. These results implied that the commonly used storage condition, drying, is not suitable for radiation and other studies.

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ESR studies of dry deoxyribonucleoprotein in the presence or absence of Ca⁺⁺ ion revealed that the addition of Ca⁺⁺ ions to deoxyribonucleoprotein resulted in a decrease of electrons transferred from protein moiety to DNA moiety.

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