This lack of a beneficial immunological effect against artificial metastases may be, at least partially, explained by the observation that internal exposures of BALB/c and C57BL/6 mice to both low (0.01 and 0.1 Gy) and intermediate (1.0 Gy) absorbed doses of the tritium-emitted radiation did not affect the systemic production of a number of pro- (IL-1β, IL-2, IL-6, TNF-α,) and anti-inflammatory (IL-1Ra, IL-4, IL-10) cytokines (Figs. 5, 6, 7) whose activities may be directly or indirectly related to the development of tumour metastases. This evidence concerns the gene IL2 and neoplasm.