Normal human PBMC were analyzed for the presence of cells expressing both T3 and NKH1 antigens, using direct two-color immunofluorescence. In six individuals, NKH1+T3+ cells were found to represent 2.5% of PBMC and 24% of the total number of NKH1+ cells. Purified NKH1+T3+ cells were shown to have the typical morphology of large granular lymphocytes (LGL). NKH1+T3+ cells also exhibited spontaneous cytotoxicity against K562 target cells and this lytic activity could be inhibited by anti-T3 mAb. Similar results were obtained with NKH1+T3+ cells cultured in vitro in lymphocyte-conditioned medium. Taken together, these results indicate that NKH1+T3+ cells represent a unique population of NK-active cells in normal peripheral blood. Although these cells exhibit LGL morphology and NK activity, this appears to be mediated through a functional T cell-like receptor for target antigen.