A biomarker that would distinguish radiation-induced biological damage from damage produced by other agents has long been a goal in radiation biology. We suggest that densely ionizing radiations such as alpha particles from radon daughters, or fission neutrons, leave a distinctive chromosomal marker that may be detected and measured long after radiation exposure. Specifically, they produce an anomalously low ratio (F) of interchromosomal to intrachromosomal, interarm exchange-type chromosome aberrations, in comparison with either X rays or chemical carcinogens. For densely ionizing radiations and for other agents, experimental values of this F ratio, determined both in vitro and in vivo, are quantitatively consistent with theoretical expectations based on considerations of chromosomal geometry and radiation track structure. The use of fluorescence in situ hybridization to measure F values in stable chromosomal aberrations, together with recent developments in techniques for harvesting viable human cells, makes the application of this biological marker quite feasible. For example, the use of this marker could greatly facilitate epidemiological studies of radon-exposed cohorts.