Chinese hamster (Cricetulus griseus) and a wide variety of cell lines derived from it have been extensively used for radiobiological and genotoxicity studies. In this report, we describe the development of chromosome-specific DNA libraries for Chinese hamster chromosomes 1, 2, 3, 5, 6, 7 and 8 by the linker/adaptor method. The specificity of these probes have been checked by FISH on primary embryonic fibroblast (CHE) metaphase chromosomes. The diploid number of chromosomes in different cell lines of CHO often varies from 19 to 21 and karyotyping is very difficult owing to many spontaneous rearrangements. Using Chinese hamster chromosome-specific DNA libraries, we have analysed the spontaneous chromosome rearrangements in three different Chinese hamster transformed cell lines, V79, CHO-KI and CHO-9. The results indicate that one of each of the first and fifth pair of chromosomes are involved in reciprocal translocations in both CHO-9 and CHO-K1 cells. The hybridisation patterns obtained with chromosome-specific probes 2, 3, 5, 6, 7 and 8 indicated rearrangements have occurred involving terminal and interstitial translocations in V79, CHO-KI and CHO-9 cells.