Although immature thymocytes from day 14 embryonic mice (FTC) express IL-2R, they have generally been considered to be unresponsive to IL-2. We show here that they can in fact undergo substantial and prolonged growth in vitro provided that high doses of IL-2 are present. The ability of FTC to grow in IL-2 could be enhanced slightly by PMA and also by IL-4, but dramatically by the combination of IL-4 + PMA with IL-2. Pretreatment of FTC with IL-4 + PMA for as little as 24 h primed FTC for rapid and prolonged responsiveness to IL-2, permitting the establishment of long term lines. Kinetic and clonal analysis revealed that most individual FTC could grow under these conditions. Although proliferating cells expressed functional IL-2R of only very low affinity, and IL-2R alpha chains were undetectable by immunofluorescence, blocking experiments showed unambiguously that both IL-2R alpha and IL-2R beta were involved in signal transmission. FTC lines and clones developed in this manner lacked lineage-specific markers of mature T cells, B cells, and myeloid cells, but expressed the NK cell markers NK1.1 and asialo-GM1. They displayed potent cytolytic activity against NK-sensitive targets, and, when stimulated with PMA+ionomycin, secreted IL-3 and IFN-gamma, but not IL-2 or IL-4. After intrathymic injection they showed no evidence of growth or differentiation. These results demonstrate that most, if not all, immature thymocytes have the capacity to differentiate into cells which appear to be indistinguishable from mature NK cells. They suggest that T cells and NK cells derive from a common precursor which in the thymic environment differentiates into T cells and in the extrathymic environment into NK cells.