A number of epidemiological and experimental data indicate that exposures to low doses of low-LET ionizing radiation may trigger the activity of natural anti-tumour immune mechanisms and inhibit tumour growth. Natural killer (NK) cells and activated macrophages play an important role in the anti-tumour defence of the host. In our experiments, BALB/c mice were irradiated with single doses of 0.1, 0.2, or 1.0 Gy X-rays and then intravenously (i.v.) injected with L1 sarcoma cells. Cytotoxic activities of NK cells and macrophages were estimated in vitro using the classical ⁵¹Cr-release and [3H] thymidine-uptake assays, respectively. The anti-asialo GM₁ (GM₁Ab) antibody and carrageenan (CGN) were intraperitoneally (i.p.) injected to block the NK cell- and macrophage-mediated activities in vivo, respectively. Whole body irradiation of mice with a single low dose (0.1 or 0.2 Gy) of X-rays led to a significant reduction of the number of tumour colonies induced in the lungs accompanied by the enhanced cytotoxic activities of both NK lymphocytes and macrophages. Treatment of mice with GM₁Ab or CGN abrogated the tumour-inhibitory effect of the exposures to 0.1 and 0.2 Gy X-rays. The obtained data suggest that suppression of the development of pulmonary tumour colonies by single irradiations of mice with the two low doses of X-rays may result from stimulation of the natural anti-tumour defence reactions mediated by NK cells and/or cytotoxic macrophages.