Human T-lymphoblasts maintained in culture for 1-2 months in T-cell growth factor conditioned medium continue growth when placed in fresh culture medium in the presence of phytohemagglutinin and irradiated adherent cells. In the absence of lectin, no growth occurs. Neither long-term cells nor irradiated adherent cells plus lectin produce soluble factors that stimulate long-term cell growth, but small amounts of this biologic activity can be detected in the culture medium when the two cell sets are mixed together. The data indicate that human long-term cell populations retain the ability to make enough TCGF to stimulate proliferation, but require physical contact with monocytes in the presence of phytohemagglutinin for this response to occur.