Because of mounting evidence of involvement of thymus-derived cells in blood formation we have studied the growth of transplanted bone marrow in mice extensively depleted of T-lymphocytes (TCD). Poor growth of parental marrow was found not to be appreciably altered in TCD hybrid recipients. However, parental thymic lymphocytes even in massive doses were not able to augment hemopoiesis in TCD hosts, in contrast to findings from sham-thymectomized or age control mice. This indication that a third (host) cell takes part in the thymocyte-marrow stem cell interaction was reinforced by the finding that isogenic (hybrid) thymocytes administered to TCD mice 5 weeks before the final irradiation restored their ability to support thymocyte-induced augmentation of parental marrow growth. Data were obtained from theta-poor sham-thymectomized irradiated controls which are interpreted as evidence for a suppressor T cell. Thymocytes administered with marrow produced a shift toward granulopoiesis in TCD mice as well as in controls. From this finding we infer that although thymus-derived cells are intimately involved in regulation of myelopoiesis, the effect of administered thymic lymphocytes on the differentiative pathway does not depend on host T cells.