In the human bone marrow the nuclear enzyme terminal deoxynucleotidyl transferase (TdT) is expressed by cells during early stages of lymphocyte differentiation. In order to investigate a possible regulation of lymphopoiesis at this level of differentiation, the relative frequency and the in vitro 3H-thymidine labeling index (3HdT-LI) of TdT-positive bone marrow cells were assessed in patients with different functional activities of the immune system. TdT-positive lymphoid precursor cells could be detected in the bone marrow of all children investigated, including six patients with various forms of immunodeficiency. Neither a transient hyperfunction of the immune system during the immunological rebound after cessation of long-term chemotherapy for acute lymphoblastic leukemia, nor a congenital or acquired hypofunction of the immune system had any detectable influence on the invariably high in vitro 3HdT-LI of TdT-positive bone marrow cells, a phenomenon possibly related to an autonomous and high turnover of this precursor cell compartment in the human bone marrow.