Synthesis of nucleic acids in blood cells in mice after irradiation with gamma rays

In the blood of mice irradiated with a single total-body dose of 8 Gy gamma rays we examined the concentration of nucleic acids and incorporation of 3H-thymidine and 14C-orotic acid. The concentration of nucleic acids, above all that of DNA in the so-called blood leukocyte mass dropped significantly in the course of the first three days after irradiation. Recovery began in the course of the second week following irradiation. On day 21 after irradiation, DNA concentration was found to be recovered temporarily and RNA concentration to exceed the control values. The increase of incorporation of 3H-thymidine confirmed an increased incidence of DNA synthesizing cells in the blood within days 12-15 after irradiation. Since the increase of incorporation of 3H-thymidine into DNA was not preceded by an increase of incorporation of 14C-orotic acid into RNA, we presume that the DNA synthesizing cells in the blood of irradiated animals do not represent the stimulated lymphocytes but circulating hemopoietic cells. Therefore, the examination of DNA synthesis in blood cells enables to record the course of bone marrow regeneration after irradiation or after application of hemopoiesis depressing substances.