Using the spleen colony-forming technique to assay hemopoietic stem cells, (CFUs) experiments have been carried out to investigate the role of thymus cells in hemopoiesis. The experiments were carried out by injecting live or killed thymus cells into lethally-irradiated mice together with hemopoietic tissue containing CFUs. The effect of thymus cells on endogenous colony formation in sublethally-irradiated mice was also investigated. Colony formation by normal bone marrow or spleen is unaffected by the addition of thymus cells, but cell populations damaged by radiation all demonstrate increased colony numbers when live thymus cells are injected within 48 hr of initiating colony formation. Endogenous colony formation resulting from an x-ray dose of 475 red is increased 2.9 times: exogenous colony formation from bone marrow or spleen populations whose CFUs content has been reduced to approximately 20% of normal with 180-rad γ-rays is increased 2.1 times. Studies investigating this enhancement demonstrate that at least some of the colonyforming cells require the cooperation of live thymus cells. Under the conditions of these experiments, at least 106 thymus cells must be injected to produce maximum enhancement.