AbstractSupernatants containing interleukin 2 (IL2) induce strong proliferation and expression of natural killer (NK)‐like activity in human thymocytes. Different supernatants were compared for: (a) IL2 activity, (b) thymocyte proliferation capacity and (c) induction of NK‐like cytotoxicity. All these activities were present in a partially purified IL2 preparation obtained by gel filtration chromatography (Mr15000–20000). However, in supernatants from different sources and in the 15000–20000‐Mrsemipurified fractions, we observed that the NK‐like cytotoxicity inducer effect did not correlate with either the mitogenicity or the IL2 activity.The presence in the supernatants of interleukin 1 (IL1), interferon (IFN) or phytohemagglutinin (PHA) does not seem to be a prerequisite for the induction of NK‐like cytotoxicity, since this activity was (a) fully present in supernatants devoid of IL1, IFN and PHA, (b) absent in preparations of IL1 and (c) not augmented after supplementation of the supernatants with IFN‐γ.We also investigated the cellular characteristics of the precursor thymocytes responsive to IL2 supernatants. Removal of the T3+cells at the initiation of the culture abrogated the proliferative response and eliminated the generation of NK‐like cytotoxicity. Under the same conditions, removal of the HTA1+population increased the proliferation and did not affect the NK‐like activity.Our results suggest that: (a) either IL2 is not responsible for the induction of NK‐like cytotoxicity or its action is modulated by other molecules, and (b) the precursor‐responder population is preferentially included