The effects of 12-O-tetradecanoylphorbol-13-acetate (TPA) on human T lymphocyte lymphocyte colony formation in vitro were investigated. The number of T lymphocyte colonies was increased 4-5 times over that of controls by the addition of TPA (10(-7) - 10(-9) M) to phytohemagglutinin (PHA)-containing cultures. Few colonies were observed when stimulated with TPA in the absence of PHA. In the cultures containing a sufficient amount of exogenous T cell growth factor (TCGF), the enhancement of T lymphocyte colony formation by TPA was not observed. TPA enhanced TCGF production by peripheral lymphocytes stimulated with PHA. The optimal concentrations of TPA for T lymphocyte colony formation were similar to those for TCGF production. These findings suggest that TPA enhanced T lymphocyte colony formation by stimulating endogenous TCGF production. Interestingly, T lymphocyte colony formation was not inhibited even at high concentrations of TPA that usually inhibit myeloid and erythroid colony formation. This difference may be due to different sensitivities to TPA between T lymphocyte colony-forming cells and myeloid and erythroid colony-forming cells.