Human T lymphocyte colony formation can be induced in a two-step culture system with phytohaemagglutinin (PHA) as the only exogenous growth factor. In this system the first step is the overnight pre-incubation of mononuclear cells in a liquid medium in the presence of PHA ('activation step'). The second step is the seeding of these PHA-activated cells into semi-solid agar, again in the presence of PHA, and their culture for 7 days ('proliferative step'). We have previously proposed that interleukin 2 (IL2) is produced during the second (proliferative) step and that this IL2 facilitates the production of colonies from PHA-activated colony-forming cells. Here we substantiate this proposal by demonstrating that both highly purified IL2 and recombinant IL2 stimulate T colony formation from PHA-treated cells. The monoclonal antibody anti-Tac, which is specifically directed against the IL2 receptor, dramatically inhibited colony formation when it was included during the agar culture (proliferative step). There was no effect when anti-Tac was included during the liquid pre-incubation culture (activation step). These results provide strong evidence that IL2 is the critical factor in the development of T lymphocyte colonies at the stage of conversion of PHA-activated cells into colonies.