The repopulation of the thymus was studied in mice after a potentially lethal dose of irradiation and injection of different numbers of syngeneic but chromosomally distinguishable bone marrow cells. The more bone marrow cells were injected, the earlier was the changeover from dividing host cells to dividing donor cells observed. At both 30 and 60 days after irradiation, the number of donor-derived T cells in the peripheral blood responding to phytohemagglutinin was directly proportional to the number of donor bone marrow cells injected, but the number of host-derived T cells was inversely proportional. Experiments in which mixtures of two syngeneic chromosomally distinguishable haematopoietic cell populations were injected after irradiation suggested that relatively small numbers of haematopoietic cells can be responsible for repopulating a single femur. Analysis of the thymus showed that the dividing cell populations within the thymus can also derive from very few precursor cells, possibly as few as one or two.