The frequencies of chromosome aberrations induced by different doses of X-rays were determined in both splenocytes and primary lung fibroblasts of Chinese hamster by bi-colour FISH using a combination of four chromosome-specific DNA libraries. The results indicate that the X-rays induced more translocations than dicentrics in Chinese hamster cells, in which the karyotype is comprised of both metacentric and acrocentric chromosomes. These results are similar to those reported in human lymphocytes, in which the karyotype contains many metacentric chromosomes. On the contrary, in mouse, which is characterized by acrocentric chromosomes only, the frequencies of translocations and dicentrics are induced in nearly equal proportions by X-rays. The ratio of translocations to dicentrics obtained in Chinese hamster cells was approximately 1.4-1.5, which supports the importance of the karyotypic features of a species in the relative induction of translocations to dicentrics. An analysis was also made on the yield of translocations and dicentrics involving individual chromosomes and the results indicate a non-random involvement of these chromosomes in the formation of aberrations.