AbstractRegulation of interleukin (IL)‐4 production, but not IL‐2 production, was found to be quite different in either freshly isolated T cells or T cell clones. Both fresh T cells and T helper 2‐like clones produced IL‐4 when stimulated with anti‐CD2 in combination with anti‐CD28. However, whereas T cell clones showed enhanced IL‐4 production when phorbol 12‐myristate 13‐acetate (PMA) was used in addition to anti‐CD2 and anti‐CD28, IL‐4 production by fresh T cells was inhibited by the presence of PMA. Prestimulation of fresh T cells led to the following observations: (a) activation in the absence of PMA led to a reversal of the PMA effect and (b) within 2 days these cells resembled T cell clones in that IL‐4 production was no longer inhibited by PMA. When prestimulation was carried out in the presence of PMA, the inhibition of IL‐4 production seemed irreversible. Removal of PMA after 3 days did not lead to renewed capability of IL‐4 production, whereas IL‐2 production was unimpaired. Our data show that the capacity of cultured T cells to produce IL‐4 is determined and fixed during