- Human hemopoietic progenitor cells have been characterized as null lymphocytes. These primitive precursors are subject to interaction with mature cells which are derived from the thymus. Such T lymphocytes exist in several subpopulations which can exhibit "helper" and "suppressor" activity in various immune responses. Evidence for the involvement of these cells in hematopathological disorders has been presented and the possibility of a physiological role for helper and suppressor T cells in hemopoiesis has been discussed. It is proposed that "steady state" blood cell formation is maintained, at least in part, by the preponderance of suppressors in the bone marrow, while the dominant helper population in peripheral blood can recirculate under the influence of corticosteroids with consequent stimulation of hemopoiesis. The influence of histocompatibility antigens on this process of cellular regulation remains to be determined.