[en] Whole-body gamma or neutron irradiation of mice with 0.01 Gy or even less causes a reduction of uptake of ³H-thymidine and ¹²⁵I-deoxyuridine into bone marrow cells, and concordantly an elevation of thymidine in the blood serum. The effects are at maximum at 4 h and subside about 10 h after exposure. To further elucidate this phenomenon, bone marrow cells were obtained at various times after whole-body exposure and incubated in vitro using different media, buffers and pH ranges. Thymidine kinase was assayed in high-speed supernatant of cell homogenates, again with different conditions of buffer and pH. Bone marrow cells from sham-irradiated mice served as controls. Tracer uptake into bone marrow cells in vitro following irradiation in vivo is depressed when the cells are collected in TC-199 medium buffered to pH 7.2-7.4 with NaHCO₃, 1.350 mg/L medium. Outside this pH range and without NaHCO₃, tracer uptake is not depressed. Concordant with depression of tracer uptake, thymidine kinase was found to be inhibited, yet beyond a dose of 0.01 Gy the effect was dose-independent and 35% of the thymidine kinase remained uninhibited. There was no inhibition when intact cells were collected outside the optimal pH range and in the absence of NaHCO₃. Addition of NaHCO₃ to cellular homogenate or supernatant failed to restore enzyme inhibition. Procain chloride given to the mice 1 h before to 1 h after irradiation fully abolished the depression of tracer uptake into cells in vitro. The effect described is produced by very few or single-cellular radiation-absorption events. It may be a physiological answer of hit cells to background radiation and may influence the effect from chronic low dose-rate exposure. (author)