Cell-mediated cytotoxicity is considered to be an important defence system of the body in the case of virus diseases, transplant rejection and in the early stages of cancer. Apart from T lymphocytes, two types of effector cells which lyse tissue culture cells by different mechanisms have been investigated intensively over the past decade--K (killer) and NK (natural killer) cells. K cells possess the capacity to lyse antibody-coated target cells. These effector cells have been described to occur mainly in peripheral blood and spleen. They are small lymphocyte-like cells which possess receptors for the Fc part of immunoglobulin G. Other surface markers may be present on these cells, depending on the organ of isolation. Other cytotoxic effector cells with similar morphology and surface markers possess the capacity to lyse tissue culture cells without participation of antibody and without in vitro or in vivo sensitization. These cells are called NK cells and are found in peripheral blood, spleen, tonsils and lymph nodes. Up to now neither type of effector cells has been unequivocably identified as mature B or T cells, granulocytes, monocytes or macrophages. Differences exist between the effector mechanisms of K and NK cells, as shown by different susceptibility to modulation of effector cell function by cortisone and, probably, by interferon. Whether K and NK cells represent functional stages of identical cells and furthermore, whether these cells belong to a third lymphocyte population or represent immature mononuclear cells are questions remaining to be clarified.