DL-alpha-Difluoromethylornithine (DFMO) is a specific irreversible inhibitor of ornithine decarboxylase (EC 4.1.1.17) and has antitumor effects. In this paper, we show that DFMO inhibits the growth of and causes severe chromosomal damage in Chinese hamster ovary cell strain A7 which grows without serum but has deficient arginase activity and therefore requires ornithine or polyamines for continuous replication. In ornithine-containing medium, the A7 cells had very few chromosome aberrations, but incubation of these cells with 0.5 mM DFMO for 7 days induced chromosome aberrations in 12 to 46% of the mitoses. Depletion of polyamines by omitting ornithine from the medium also caused chromosome aberrations. The chromosomal damage found after DFMO treatment alone and ornithine deprivation alone were of similar nature. In addition to chromosome breaks, there were chromosome fragmentation and structurally changed chromosomes including rings, chromatid exchange configurations, and chromosome elongations. A phenomenon resembling premature chromosome condensation was also seen. Double-minute chromosomes were visible in some mitoses, and the chromosome elongations sometimes gave an impression of homogeneously staining regions.