Peripheral blood lymphocytes (PBLs) from healthy donors were more than 95% enriched for γδ T cells. Vγ9/Vδ2 was the predominant T-cell subset and represented more than two thirds of the isolated γδ T cells. When activated by surface-immobilized anti-TCR51 monoclonal antibody (MoAb) during 5 days of culture, 76 T cells demonstrated a significant fourfold to 15-fold stronger cytolytic activity against various tumor cell lines than did PBL polyclonal αβ T cells analogously activated by surface-immobilized anti CD3 MoAbs. Blocking experiments with specific MoAbs demonstrated that the cytolytic activity of 76 T cells was non-major histocompatibility complex (MHC) restricted and did not involve the γδ5 T-cell antigen receptor (TCR) but that cytolytic activity was dependent on LFA-1β/ICAMl interaction. The proliferative responses of 78 and αβ T cells to Ovcar-3 tumor cells and irradiated allogeneic PBLs were inhibited by anti-HLA-A, B, C MoAb. Our findings suggest that γδ T cells activated via the TCR have a significant advantage in nonMHC-restricted lysis of various tumor cell lines over PBL αβ T cells stimulated analogously with anti-CD3, which may be important in terms of applicability of activated γδ cells for adoptive immunotherapy of metastatic cancers. [J Natl Cancer Inst 83: 1564–1569, 1991]