The colony forming capacity of bone marrow cells from thymectomized mice was shown to be reduced as compared to that of bone marrow cells from normal donors. A further indication of changes in the proliferative capacity of colony forming cells (CFU-S), following thymectomy, was given by examination of the sensitivity of these cells to chlorambucil and by 3H-thymidine 'suicide' experiments; both showed that CFU-S from thymectomized mice were not cycling at the same rate as normals. It was also found that in late pregnancy of thymectomized females, there is an elevation in the number of bone marrow CFU-S and an increase in cell cycling. Such an increase could also be achieved by implantation, into thymectomized mice, of thymus lobes in closed diffusion chambers. Finally, in vitro administration of a thymus hormone (THF) reversed the suppressive effect of thymectomy on DNA synthesis in bone marrow CFU-S. Since the action of THF was restricted to bone marrow cells of thymectomized mice it is plausible that normal bone marrow contains at least two subpopulations of CFU-S, one of which is dependent upon a humoral product of the thymus.