This paper reviews the experimental results showing that a prior exposure to a low dose of ionising radiation induces an adaptive response expressed as a reduction of gene mutation in various cell systems. The data show that the mutagenic adaptation shares common features with the clastogenic adaptation, i.e., priming dose level, kinds of conditioning agents, time interval between conditioning and challenging treatments, degree of induced protective effect (40-75%), transitory response and inhibition by 3-aminobenzamide, a DNA repair inhibitor. Moreover, the deletion-type mutations are predominantly reduced in adapted cells, suggesting that the mechanism underlying mutagenic adaptation preferentially facilitates the removal of the DNA lesions leading to deletion-type mutations. These lesions are thought to be double-strand breaks which are likely to be also involved in the production of chromosomal damage. Recent findings on the molecular processes implicated in the cellular response to radiation provide some clues for the mechanisms that could be triggered by low-dose exposure and ultimately contribute to the protective effect. There is some evidence that the protein kinase C-mediated signalling pathway is a key step for the transduction of the low-dose-induced signal. Several recent reports indicate that the low-dose triggers changes in the expression of several genes whose products, though most of them are still not identified, would be related to DNA repair and/or control of cell cycle progression.