The possibility of the generation of secondary cytotoxic T lymphocyte (2 degrees CTL) activity from alloantigen primed T cells by genetically homogeneous recombinant human interleukin 2(G-IL2) was analyzed. Not only purified IL2 by cell culture (C-IL2) but also G-IL2 induced 2 degrees CTL activity from primed T cells generated in mixed lymphocyte culture. This induction process required RNA and protein synthesis, while DNA synthesis was not relevant. In parallel to the induction of 2 degrees CTL activity, transition of the primed cells from the resting G1a to activated G1b has taken place. 2 degrees CTL activity was induced from nylon column-purified primed T cells in the absence of accessory cells and also in the absence of Lyt-1+ T cells. Interferon-gamma (IFN-gamma) induction was detected during the course of activation of alloantigen-primed T cells into the cytotoxic state. When IL2 was absorbed, IFN-gamma in the culture supernatant did not induce 2 degrees CTL activity. These results suggest that the IL2 molecule possessing T cell growth factor properties shares the nature of differentiation factor in terms of the activation of alloantigen-primed T cells into cytoxic state. The role of IFN-gamma produced in situ for this activation process awaits further investigation.