CBA/H male mice have been used to study the recovery of haemopoietic stem cells following 3 Gy whole-body X-irradiation. Despite significant proliferation haemopoietic stem cells (measured as spleen colony-forming units or CFU-S) did not recover to control values and their numbers remained sub-optimal at 600 days post-irradiation. At this time there were significant increases in both proliferation and self-renewal characteristics of the CFU-S but no impairment of the capacity of bone marrow cells to rescue and repopulate lethally irradiated transplant recipients.