[en] When investigating the biological effects of ionizing radiation on the haemopoietic system, a confounding problem lies in possible differences between the biological effects of sparsely ionizing, low linear energy transfer radiation such as X-, β- or γ-rays, and densely ionizing, high linear energy transfer radiation such as α-particles. To address this problem we have developed novel techniques for studying haemopoietic cells irradiated with environmentally relevant doses of α-particles from a plutonium-238 source. Using a clonogenic culture system, cytogenetic aberrations in individual colonies of haemopoietic cells derived from irradiated stem cells have been studied. Exposure to α-particles (but not X-rays) produced a high frequency of non-clonal aberrations in the clonal descendants, compatible with α-emitters inducing lesions in stem cells that result in the transmission of chromosomal instability to their progeny. Such unexpected instability may have important implications for radiation leukaemogenesis. (author)