NK1.1+ CD4+ T cells produce more IL-4 and IFN-gamma in response to stimulation with anti-CD3 than conventional CD4+ T cells that have been primed to be, respectively, Th2 or Th1 cells. Furthermore, NK1.1+ CD4+ T cells produce IL-4 even if cultured in the absence of IL-4, whereas conventional CD4+ T cells require IL-4 to develop into IL-4 producers. The IFN-gamma-producing capacity of NK1.1+ CD4+ T cells is enhanced by IL-12. In addition, NK1.1+ CD4+ T cells produce substantial amounts of IL-3, IL-5, and IL-10 upon anti-CD3 stimulation. NK1.1+ CD4+ T cells can be stimulated to produce IL-4 by culture with L cells expressing CD1 (L-CD1), but primary IL-4 production was rather slow and weak. Restimulation of L-CD1-activated NK1.1+ CD4+ T cells with L-CD1 gave rise to a much stronger and more rapid response, yielding IL-4 production comparable with anti-CD3 activation of cells initially primed with anti-CD3. L-CD1 stimulation of L-CD1-primed cells resulted in far less IFN-gamma than that elicited by anti-CD3 from cells that had been primed with anti-CD3, but such production is substantially increased by adding IL-12 to the culture. The differing patterns of cytokine production by NK1.1+ CD4+ T cells suggest that these cells may have complex effects on the priming of conventional T cells and may not simply drive such cells to the acquisition of a Th2 phenotype.