A human-hamster hybrid cell line containing only the human X chromosome (GM06318B) was exposed to 6,000-7,000 rad of X-rays and fused with a mouse cell line (CL1D,TK-). Three radiation hybrids, LXKC40, LXKC50, and LXKC56, were selected among 39 independent clones containing human material. Two-color in situ hybridization with total genomic DNA probes (cot1 human DNA and hamster total genomic DNA) was used to analyse the irradiated chromosome rearrangements. With this three-species model system (human-hamster-mouse) and the chromosome painting process it was possible to determine the origin of each chromosomal fragment in metaphase and interphase. The results obtained indicate preferential rearrangement between irradiated human and hamster chromosomes. Whole, apparently intact hamster chromosomes were observed in all the mitoses. We suggest that these chromosomes could be neoformated from random fragments after irradiation. Hamster and human "minichromosomes" were also detected. While the integration of human material into the mouse genome was exceptional, the integration of hamster material into mouse chromosomes was more frequent. During interphase the irradiated chromosome domains were often at the periphery of the nucleus. Irradiated material protruded at the periphery of the nuclei. Micronuclei containing hamster material were detected in the vicinity of these protrusions.