The ability of the (T2G4)n telomeric repeats to maintain a linear structure in an extrachromosomal replicating plasmid in mouse C127 cells was tested in a vector based on Bovine papillomavirus type-1, ARS, HIS3 and the neo genes. Digestion with BamHI releases a BPV-1 containing fragment with a (T2G4)n repeats at each end which was introduced to yeast and microinjected into mouse C127 cells. While the linear construct was maintained as extrachromosomal structure in yeast cells, none of the resulting G418-resistant mouse cells transformants were found to have extrachromosomally replicating linear plasmids. Analysis of transformed mouse C127 DNA suggested that in some, the linear fragment had integrated into mouse chromosomes, whereas in other cell lines the fragment may have circularised and possibly been replicating extrachromosomally as a high molecular weight structure. In some of the mouse transformants the (T2G4)n repeats had been deleted from retained plasmid sequences.