A quantitative assay for the hematopoietic precursor of thymocytes has been developed. Using this assay the kinetics of appearance of the progeny of transfused bone marrow and spleen cells in the thymus of irradiated (760 R) mice has been studied. Precursor cells are seven to eightfold more common in bone marrow than in spleen and are absent from peripheral lymph nodes. They decline in number as the animals age. When hematopoietic cells are injected immediately after lethal irradiation only a small number of cells actually enter the gland. Their progeny are not detectable in the thymus for 8-12 days. The time of their detection depends both upon the size of the residual endogenous thymocyte population and the number of progenitor cells injected. Evidence has been presented that excludes thymic injury as the basis for the delay in the appearance of donor type cells and indicates that neither the production of a "homing" signal in the irradiated animal nor the development of precursor cells are limiting factors in the rate of thymic repopulation. These studies indicate that only an exceedingly small number (less than 100) of prothymocytes are required to repopulate the thymus of an irradiated mouse. This restricted number of progenitors must produce the entire repertory of T-cell immunologic responsiveness seen in the first weeks after repopulation.