The specific and natural killer (NK)-restricted nature of autologous tumor killing by blood lymphocytes was studied in patients with carcinomatous pleural effusions. Large granular lymphocytes (LGL) and small T-lymphocytes were isolated by centrifugation on discontinuous Percoll density gradients. Tumor cells freshly isolated from pleural effusions of cancer patients were classified according to their susceptibility to purified LGL from normal donors in a 4-hour 51Cr release assay. Of 15 NK-sensitive tumors, 14 were lysed by fresh autologous LGL, whereas only 2 were killed by T-cells. Neither LGL nor T-cells were cytotoxic to NK-resistant autologous tumor. LGL and T-cells were then cultured in vitro with autologous tumor cells for 6 days. In 13 of 15 autologous mixed lymphocyte-tumor cultures (MLTC) NK-sensitive tumor-cultured LGL maintained their autotumor killing activity, whereas LGL cultured alone lost the activity. Depletion of high-affinity sheep erythrocyte-rosetting cells from Percoll-purified LGL resulted in an enrichment of effector cells. LGL from autologous MLTC were able to kill NK-susceptible allogeneic effusion tumor and K562 as were fresh LGL. No lysis of NK-resistant autologous tumor was observed with cultured LGL. In contrast, activation of T-cells in autologous MLTC resulted in the generation of autotumor killer cells in 10 of 15 NK-sensitive and 3 of 6 NK-resistant tumor samples. However, cultured T-cells were incapable of killing allogeneic tumor and K562. In autologous MLTC T-cells proliferated in response to autologous tumor, whereas no proliferation was observed in the culture of LGL. The enrichment of blasts from cultured T-cells on discontinuous Percoll gradients induced an augmentation of autotumor cytotoxicity, with no reactivity in blast-depleted, small, resting T-lymphocytes. These results indicated that 2 distinct types of autotumor-recognizing lymphocytes, LGL and T-cells, are present in the peripheral blood of cancer patients.