Abstract Interest in the possibility of growing large numbers of antigen-reactive human T lymphocytes led to a thorough study of the production of T-cell growth factor(s) that allow the continued proliferation of human T lymphocytes in culture. Such factors are found in the supernates of human peripheral blood lymphocytes stimulated with phytohemagglutinin. Whereas the growth factor from a commercial laboratory was produced with pooled lymphocytes from many donors, we developed supernates from mononuclear peripheral blood leukocytes of single donors. The present study has revealed several important points for production of T-cell growth factor: (i) the commercial production was unnecessarily complex, and could be simplified; (ii) stimulation of leukocytes with phytohemagglutinin was needed, and could not readily be replaced by Con A or alloantigens; (iii) some normal donors were high producers of T-cell growth factor, whereas others were consistently low producers; (iv) adherent cells, probably monocytes, could be suppressive in this system. A synergistic effect between the stimuli of phytohemagglutinin and of cells from certain B lymphoblastoid cell lines was also found.