Supernatants containing interleukin 2 (IL 2) induce strong proliferation and expression of natural killer (NK)-like activity in human thymocytes. Different supernatants were compared for: (a) IL 2 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 (Mr 15 000-20 000). However, in supernatants from different sources and in the 15 000-20 000-Mr semipurified 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 (IL 1), 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 IL 1, IFN and PHA, (b) absent in preparations of IL 1 and (c) not augmented after supplementation of the supernatants with IFN-gamma. We also investigated the cellular characteristics of the precursor thymocytes responsive to IL 2 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 IL 2 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 in the mature T3+ subset.