AbstractWe analyzed the release of activities capable of stimulating the in vitro growth of human hemopoietic progenitor cells by long‐term cultured T cell growth factor (TCGF)‐dependent human T lymphocytes. Seven cell lines tested produced colony‐stimulating activity (CSA) as well as burst‐promoting activity (BPA). The CSA stimulated primarily the growth of the cells forming colonies after 14 days of incubation. In addition the supernatants from these seven Tcell lines showed the ability to induce the in vitro growth of mixed granulocyte, erythroid, megakaryocyte, macrophage colonies (CFU‐GEMM). The release of hemopoietic factors did not depend on the presence of accessory cells or phytohemagglutinin or serum during the incubation for factor production. In six of the T cell lines the majority of the cells were reactive to the OKT 8 monoclonal antibody (MoAb), whereas one cell line contained mostly OKT 4+ cells. Suppressor activity was detected in three tested OKT 8+ cell lines, while the one OKT 4+ displayed helper activity. All cell lines produced hemopoietic factors with equal efficiency. These results indicate that factors affecting human hematopoiesis are produced by normal T lymphocytes in long‐term culture and this property is not related to the helper or suppressor activity of the cul