AbstractAmong the cytotoxic agents which particularly cause damage to cell renewal systems, ionizing radiation is one of the most effective ones since it leads to inactivation of all types of proliferating cells including resting stem cells. It is the aim of this paper to present the effects of continuous low dose rate total body irradiation (TBI) on hemopoiesis in dogs. The animals were exposed to γ‐rays from a 60‐Co source, receiving a daily radiation dose of 0.0188 Gy for indefinite times. Sequential hematological studies performed included determinations of peripheral blood cell counts and of total cell numbers in standardized bone marrow samples, assessments of progenitor cells GM‐CFC in the blood and bone marrow, and of colony‐stimulating activity (CSA) in the serum. The lymphocytes, the thrombocytes and neutrophilic granulocytes uniformly showed early decreases within the first 200 to 500 days corresponding to cumulative radiation doses in the range up to 3.8 to 9 Gy, but remained stable at subnormal levels in the period up to 1,700 days of exposure. The GM‐CFC numbers in bone marrow samples from the rib clearly showed a strong decrease within the first 150 days of exposure preceding the changes in the blood granulocyte concentration. A transient partial recovery of the GM‐CFC was observed at later times between 700 and 1,200 days of exposure, followed by another decrease to extremely low values at cumulative doses in the range of 32 Gy. Subnormal numbers of GM‐CFC in the peripheral blood within the whole period of exposure clearly signaled persistent alterations in the progenitor cell compartment, whereas increased levels of CSA in the serum were indicative of compensatory mechanisms operating under the influence of humo