Natural killer (NK) cells are large granular lymphocytes (LGL) whose functional characteristics play an important immunological role in tumor surveillance, bacterial and viral infections, baematopoiesis, and tissue transplantation. NK cell cytotoxicity is mediated through non-MHC-restricted lysis and by antibody-dependent cellular lysis against NK-sensitive and insensitive targets. Non-MHC restricted lysis of targets can occur when NK cells are in a non-activated state (inherent NK cell cytolytic activity) and following activation by interleukin 2 (IL-2).