[en] The expression of G-CSF receptors in bone marrow cells (BMC) was studied by using ¹²⁵I labelled G-CSF (¹'2⁵I-G-CSF). The results showed that the reaction equilibrium of ¹²⁵I-G-CSF bound to BMC at 37 degree C was reached in 60 minutes, the maximum binding (Bmax) to 3 x 10⁶ BMC being 15.1 pmol, the dissociation constant (Kd) being 78.6 pmol, and the estimated number of G-CSF receptors per cell being about 3100. The number of G-CSF receptors in BMC in mice 48 hours after whole body exposure to doses of 50, 75 and 100 mGy X-rays increased significantly to 161%, 169% and 342% of controls, respectively. The results suggested that the expression of G-CSF receptors in BMC was enhanced markedly following low dose radiation, which would lead to stimulation of BMC proliferation