N lymphocytes, like interferons, have three major biological functions: (1) antitumor activity; (2) antiviral activity; and (3) immunoregulatory activity. Antitumor activity is also referred to as natural killer (NK) activity which represents the non-MHC-restricted lysis of NK-sensitive tumor cells by effector cells obtained from a nonimmunized host. N lymphocytes play a pivotal role in a network of immune functions. The review discusses the (1) properties of natural killer activity, (2) importance of natural killer activity against tumor cells, (3) importance of N lymphocytes in viral infections, (4) regulation of natural killer activity by cytokines, (5) alteration of natural killer activity by suppressor cells, and (6) alteration of natural killer activity by chemicals.