[en] Radiosensitivity of spleen CFU depositing in bone marrow and in the spleen does not differ essentially. For CFU forming colonies in the spleen the Dsub(o) value varies within 105-120 R. For CFU forming colonies in bone marrow the value of Dsub(o) is 120-135 0. It is suggested that in bone marrow there are 2 fractions of CFU one of which is radiosensitive and the other radioresistant. It is mainly radiosensitive fraction of CFU that is localized, in the spleen