Epidemiological studies have suggested, but not demonstrated, a role of exposure to 50/60-Hz magnetic fields in increasing cancer risk in man (workers and the general population). A possible target of magnetic fields is the immune system. In particular, it is known that an important defence against cancer is represented by natural killer (NK) cells capable of killing cancer cell targets. To test this hypothesis, human NK cells, stimulated or not with phytohaemagglutinin or interleukin 2, were exposed to 50-Hz sinusoidal magnetic fields before or during the cytotoxicity test, and then mixed with a variety of target cancer cell lines (Daudi, Raji, U937, H14, IGROV, SW626, K562, HL60). The experiments were performed in two laboratories (Rome and Modena) by means of two different exposure systems. The results of both laboratories suggest that 50-Hz sinusoidal magnetic fields with flux densities up to 10 mT do not affect the cytotoxic activity of human NK cells.