Non-specific cytotoxic cells (NCCs) in teleosts and their evolutionary homologue are a subpopulation of lymphocytes with properties that distinguish them from either B- or T-cells. One such property is that NCC/natural killer (NK)/lymphokine activated killer (LAK) cells express spontaneous, non-major histocompatibility complex restricted cytotoxic activity. NCC and LAK lyse a variety of transformed murine and human B-cell, T-cell and myeloid targets. A 32 kDa membrane protein [non-specific cytotoxic cell receptor protein (NCCRP-1)] expressed by NCC and certain mammalian NK/LAK cells mediates this cytotoxicity. NCCRP-1 is evolutionarily conserved and is found in species ranging from marine and freshwater teleosts to higher mammals.