Recently, we have reported a novel protocol for the generation of highly efficient cytotoxic effector cells by culturing PBLs in the presence of IFN-gamma, IL-2, mAb against CD3, and IL-1 alpha. We have termed these cultures cytokine-induced killer (CIK) cells because the phenotype of the cells with the greatest cytotoxicity expresses both the T cell marker CD3 and the NK cell marker CD56. Cells with this phenotype are rare (approximately 1 to approximately 5%) in uncultured PBLs. CD3+CD56+ cells expand nearly 1000-fold under these culture conditions. The majority of the CD3+CD56+ cytotoxic cells in CIK cultures were derived from CD3+CD56- T cells, and not CD3-CD56+ NK cells. Expression of CD56, but not CD8, on CD3+ cells correlated with the greatest cytotoxicity against various cellular targets. We have used mice with severe combined immunodeficiency (SCID) injected with human lymphoma cells to evaluate the in vivo antitumor effects of CIK vs lymphokine-activated killer (LAK) cells. Groups of animals inoculated with 1 x 10(6) SU-DHL4 cells (a human B lymphoma cell line with a t(14;18) chromosomal translocation), injected 1 day later with CIK cells either i.v. or i.p., had significantly prolonged survival compared with control animals injected with tumor cells alone (median survival 90 days vs 58 days, p < 0.001) or animals treated with LAK cells (median survival 90 days vs 68 days, p < 0.002). Approximately 30% of the SCID mice challenged with SU-DHL4 cells and treated with CIK cells became long-term survivors compared with none of the animals treated with LAK cells. No molecular evidence of occult lymphoma was found in the CIK cell-treated long-term survivors when their bone marrow, spleen, liver, and lung were analyzed by t(14;18) PCR at the end of 6 mo. By using these culture conditions, a novel population of cytotoxic cells can be generated readily from T cells that have superior in vivo antitumor activity in SCID mice, as compared with LAK cells.