We have previously reported non-clonal cytogenetic aberrations in the clonal descendants of murine haemopoietic stem cells irradiated in vitro with alpha-particles. The data are consistent with a transmissible chromosomal instability induced in a stem cell resulting in a diversity of aberrations in its clonal progeny many cell divisions later. To determine whether this instability is transmissible in vivo, bone marrow cells obtained from the male mouse have been irradiated with alpha-particles in vitro and transplanted into female recipients. In the repopulated haemopoietic system we have observed persisting chromosomal instability up to 1 year post-transplantation. The findings demonstrate a long-lived effect of the alpha-particle-induced lesion in the donor repopulating stem cells.