NK1.1+ CD4+ T cells produce IL-4 promptly in vivo upon injection of anti-CD3 and may play a role in initiating Th2 cell-mediated immunity. To characterize their in vitro activation properties, NK1.1+ CD4+ T cells were obtained in high purity from spleens of normal C57BL/6 mice, where they represent 2 to 5% of CD4+ T cells, or from MHC-class II I-Ab gene knockout mice, where they constitute 42% of CD4+ T cells. Activation of NK1.1+ CD4+ T cells from either source with plate-bound anti-CD3 resulted in loss of expression of NK1.1 as determined both by flow cytometric analysis and by reverse transcriptase-PCR analysis. A portion of these cells also lost CD4 expression. Both the CD4+ and CD4- activated cells retained the over-representation of V beta8 and V alpha14 chains and expressed the intermediate levels of the TCR-CD3 complex that is characteristic of resting NK1.1+ CD4+ T cells. The anti-NK1.1 mAb used for cell sorting was not the cause of NK1.1 or CD4 disappearance, since the sorted cells remain both NK1.1+ and CD4+ when cultured in the absence of anti-CD3 or in the presence of anti-CD3 and cyclosporin A. Furthermore, NK1.1+ CD4+ T cells that were not treated with anti-NK1.1 Ab also lost NK1.1 expression after activation. Populations of activated CD4+ and CD4- cells (derived from NK1.1+ CD4+ T cells) produced both IL-4 and IFN-gamma upon restimulation with plate-bound anti-CD3.