Lymphokine-activated killer (LAK) cells have been defined as interleukin 2 (IL-2)-activated cytolytic effector cells exhibiting non-MHC restricted killing against a wide range of NK-sensitive and NK-resistant tumor cells. There has been considerable debate as to whether LAK cells are derived from NK cells or from a unique precursor population. In the present study, we compare LAK cells derived from T-cell-depleted nylon-wool-non-adherent spleen cells with endogenous NK cells and NK cells activated with the interferon inducer polyl.C., in terms of their phenotype and functional characteristics. The predominant splenic LAK precursor in the mouse was found to be a nylon-wool-non-adherent, thy1-, MICG-, J11d.2-, asialo GM1+ cell. This phenotype is shared by endogenous NK cells. A significant number of activated NK cells express macromolecular insoluble cold globulin (MICG) in addition to asialo GM1. Neither endogenous nor activated NK cells express a heat-stable antigen found on bone-marrow cells, immature T cells, and most B cells and defined by monoclonal antibody (MAb)J11d.2. However, J11d.2 is expressed on some LAK precursor and effector cells. The asialo GM1 marker is common to all LAK effector cells, while many are also thy1+, and/or MICG+. LAK effector cells are therefore a heterogeneous population sharing some phenotypic characteristics in common with NK cells. In addition, there is a positive correlation between LAK and NK activity in "high NK" and "low NK" mouse strains, suggesting that NK cells and LAK cells share a common lineage. Monoclonal antibodies to the alpha and beta chains of LFA-I inhibit LAK and activated NK function, while endogenous NK and CTL killing is affected only by anti-alpha chain antibodies. LAK cells, like MHC-restricted and non-restricted CTL clones, express mRNA transcripts of the C11 serine protease gene. We conclude that LAK cells share several features in common with cells of the NK lineage and may therefore represent NK cells in a unique state of activation. LAK cells appear to employ cytolytic machinery common to other lytic cell types.