The results of our previous study indicate that stimulated natural killer (NK) lymphocytes and cytotoxic macrophages are involved in retardation of the development of syngeneic tumour nodules in mice pre-exposed to low-level irradiations with X-rays. In the present investigation we examined the selected parameters of these cells activity that may be responsible for the X-ray-induced enhancement of the NK cell- and macrophage-mediated cytotoxicity. BALB/c mice were whole-body irradiated (WBI) with a single dose of 0.1, 0.2, or 1.0 Gy of X-rays. Cytotoxic activities of NK cells and macrophages were estimated in vitro using the classical 51Cr-release and [3H] thymidine-uptake assays, respectively. Colorimetric assay with the Griess reagent was used for estimation of the nitric oxide (NO) synthesis. Surface expression of the Fas ligand (FasL) on NK splenocytes was determined using the anti-FasL antibody. The levels of interferon-γ (IFN-γ) and tumour necrosis factor-a (TNF-α) were examined in the incubation media of the collected NK cells and macrophages, respectively, using the ELISA assays. The results indicate that the low-level radiation-induced enhanced cytotoxicity of the two cell types coincided with the elevated expression of FasL on the NK lymphocytes and the increased production of NO and TNF-α in the activated macrophages.