The behaviour of telomeric repeat sequences in Chinese hamster CHO and CHE cell lines treated with the radiomimetic drugs bleomycin (BLM) and streptonigrin (STN) and the effect of these drugs on telomerase activity was investigated. Fluorescence in situ hybridisation revealed that 18% of the scored aberrations induced by BLM and 14% of those induced by STN in CHO cells exhibited telomeric repeat signals. In CHE cells, 29% of the total aberrations induced by BLM and 45% of those induced by STN involved telomeric repeat sequences. Acentric fragments labelled along their entire length and translocations of telomeric repeat sequences were also found in both cell lines. These results suggest that telomeric repeat sequences are preferentially involved in chromosome breakage, fragility and recombination induced by radiomimetic agents. In addition, some of the damaged CHE cells exhibited one or more chromosomes with additional zones of hybridisation, indicating the possible amplification of (TTAGGG)(n) repeats by telomerase. However, the fact that none of the radiomimetic compounds tested produced any effect on telomerase activity suggests that this enzyme is not related to the assumed amplification events induced by BLM and STN in CHE cells.