Cytokine-induced killer (CIK) cells are highly efficient cytotoxic effector cells capable of lysing tumor cell targets. Cultures of human CIK cells have been shown to have enhanced cytotoxicity and to proliferate more rapidly than lymphokine activated killer (LAK) cells by both in vitro and in vivo studies. In this report, we have further characterized the phenotype of CIK cells and explored the molecular structures involved in CIK-mediated cell lysis of tumor target cells. The dominant cell phenotype in CIK cell cultures expresses the alpha, beta T cell receptor (TCR-alpha/beta). In addition, CD56 is expressed on the main effector cell on a per-cell basis. Interestingly, the total number of CD56+ cells increases more than 1000-fold during the generation of CIK cells, mainly due to expansion of CD56+ cells coexpressing CD3. The higher lytic activity of CIK cells as compared to LAK cells is mainly due to the higher proliferation of CD3+CD56+ cells and to the cytotoxic activity of TCR-alpha/beta+ cells in CIK cell cultures. CIK-mediated cellular lysis is non-major histocompatibility antigen (MHC) restricted. The cytotoxic effect of CIK cells against tumor targets is blocked by antibodies directed against lymphocyte function-associated antigen (LFA-1) and its counter receptor, intercellular adhesion molecule-1 (ICAM-1).