This work studied the effect of the lectin phytohaemagglutinin (PHA) and bacterial lipopolysaccharide (LPS) on the activity of nonspecific cytotoxic cells (NCCs) and macrophages of peripheral blood in the rainbow trout. NCCs are cells of the innate immunity, homologues to Natural Killer (NK) cells of mammals, capable of eliminating cells infected with intracellular pathogens and also tumoral cells. Their lytic activity is mediated by the perforin/granzyme system and the Fas receptor-Fas ligand (FasR-FasL) system. Activity of NCCs was evaluated by their capacity to lyse HL-60 tumor cells. On the other hand, macrophages eliminate microorganisms by several mechanisms, including the production of reactive oxygen species (ROS) and nitric oxide (NO). Results showed that PHA significantly increased NCCs cytotoxic activity against HL-60 cells, even under conditions in which the perforins/granzymes lytic system should be suppressed, supporting the existence of the FasR-FasL system in trouts. PHA showed no effect on ROS and NO production by macrophages. Finally, LPS showed no effect on the cytotoxic activity of NCCs or in the production of ROS and NO by macrophages.