LAK are cytolytic lymphocytes with the unique capacity for killing NK-resistant fresh human tumor cells in short-term assays. LAK kill autologous as well as allogeneic tumors with complete cross-reactivity. Initial studies on the classification of LAK conclude that LAK are distinct from the classical NK and T lymphocyte systems, based on a number of criteria including surface phenotype, activation conditions, and a spectrum of susceptible target cells. LAK kill ras oncogene-transfected fibroblasts like they kill fresh tumors. As yet, the target cell determinant responsible for susceptibility to LAK lysis is unknown. Activation of LAK requires only IL 2 and is blocked by monoclonal antibodies to the IL 2 receptor. Because only IL 2 alone is sufficient for LAK activation, we have done in vitro testing to determine whether fresh PBL could be activated in the presence of tumor, as might be desirable in vivo. LAK were activated sufficiently to mediate significant destruction of fresh tumor. We also tested whether LAK could be maintained in the presence of large tumors, providing IL 2 was added. Again, results were positive, suggesting that LAK either recycle or are a self-renewing population that depend on IL 2 for continued functions. Because of these and other findings, we have initiated a clinical protocol to test whether LAK made from the PBL of patients with brain tumor could eliminate residual glioma tumor cells. Autologous LAK plus rIL 2 to maintain lytic ability are injected during surgery. Preclinical studies in a rat glioma model have shown this approach to be safe, and previous in vivo murine studies have concluded that LAK kill tumors in Winn-type lung colony formation tests (Kedar et al. 1982). Much work is needed before we can understand the LAK phenomenon and determine its usefulness in cancer therapy, as well as its inherent biologic role. We hope that this chapter will stimulate both interest and the basic research needed to realize LAK potential.