The morphologic and functional characteristics of cells freshly isolated from human peripheral blood and bearing a T cell receptor (TcR) gamma/delta were analyzed. Cell preparations highly enriched for TcR gamma/delta+ cells were obtained by treatment of E rosette-forming lymphocytes with anti-CD4 and anti-CD8 monoclonal antibodies (mAb) and complement. These preparations consisted of 64-82% TcR gamma/delta+ lymphocytes, as indicated by the sum of cells reacting with the BB3 and A13 mAb which define two distinct, nonoverlapping, TcR gamma/delta+ cell subsets in the peripheral blood. TcR gamma/delta cells were able to form conjugates with the natural killer-sensitive K-562 and with the natural killer-resistant HL-60-R tumor cell lines. The cytochemical localization of lysosomal acid hydrolases showed that 95%-98% of the cells in the TcR gamma/delta+ preparations had the morphologic features of granular lymphocytes. Moreover, electron microscopy analyses showed that TcR gamma/delta+ cells had electron-dense granules dispersed in the cytoplasm and a variety of smooth vesicles, a morphology identical to that of other CD3- or CD3+ granular lymphocyte subsets. Freshly isolated TcR gamma/delta+ cells were unable to lyse K-562 and natural killer-resistant targets, such as HL-60-R and P815. However, low levels of target cell lysis were observed upon triggering of the effectors by anti-CD3 TcR mAb or by lectin. After short-term culture with interleukin 2, TcR gamma/delta+ cells acquired a strong cytolytic activity against K-562 and HL-60-R target cells in the absence of triggering stimuli, and also displayed high levels of cytolytic activity against P815 in the presence of anti-CD3/TcR mAb.