CD3+ WT31- T cells were sorted from peripheral blood of a normal healthy donor by a FACS IV and cloned by limiting dilution in the presence of a phorbol ester (tetradecanoyl phorbol acetate, TPA), calcium ionophore (ionomycin, Io), interleukin-2 (IL-2), allogeneic cells, and phytohemagglutinin (PHA). One of the derived clones, 290-2, was investigated in detail. 290-2 mediated strong natural killer (NK) but not lymphokine-activated killer (LAK) activity. It proliferated in the presence of IL-2 but not IL-4. It carried the surface phenotype CD3+ WT31- CD4weak+ CD8-, CD16-, and Leu 19+. Expression of CD4 was heterogeneous within the clone, since two of three subclones were also CD4weak+ but one was CD4-. NK activity was blocked by monoclonal antibody (moAb) to CD1 1a (LFA1), but not by monoclonal antibody (moAb) to either CD3 or CD4. Northern blotting revealed T-cell receptor (TCR-gamma) but not alpha- or full-length beta-chain mRNA. 290-2 proliferated autonomously when stimulated with a combination of TPA +Io, with PHA or CD3 moAb and autologous B-cell lines (B-LCL) (and this was inhibited by an anti-IL-2 receptor moAb), but not to allogeneic B-LCL or any of the other stimulating agents alone. Unexpectedly, the TPA + Io stimulus which resulted in maximal proliferative responses did not trigger interferon-gamma or granulocyte/macrophage colony-stimulating factor production, although both lymphokines were secreted in the presence of B-LCL + TPA + Io. Proliferative responses were not enhanced by the presence of B-LCL. Thus, activation signals sufficient for autocrine proliferative responses were insufficient for secretion of other lymphokines. Such clones will provide valuable reagents for investigating the biology of the TCR-gamma+ T cell.