We have studied the response of the mouse haemopoietic system to 5-fluorouracil, to shielded-limb irradiation and to total-body irradiation plus bone marrow injection. We have also studied the response to a dose of 5-FU given 10 weeks subsequent to each of these treatments. After 5-FU, the CFUs number rises rapidly reaching levels well above normal 11 days after treatment but the GM-CFC numbers increase more slowly and remain subnormal. By contrast, the GM-CFC number reaches normal 3-4 weeks after partial-body irradiation or irradiation/reconstitution although the CFUs number rises slowly and does not reach pre-treatment levels. Two possible explanations of the difference are discussed; one is that irradiation damages the haemopoietic environment in a way which prohibits fast proliferation of CFUs, the other that 5-FU inhibits the differentiation of CFUs into committed precursors. No difference which could be attributed to the exposure of stem cells to artificial media was seen between the responses to partial-body irradiation and to transplantation. The response to a subsequent dose of 5-FU was the same in all groups indicating that radiation damage to the stroma is either non-existent after 850R, or is repaired within 10 weeks, or is irrelevant to the recovery from 5-FU.