It has been recently reported that the pR plasmid enhances the UV survival in E.coli c600. In order to test whether this function may be expressed also in mammalian cells, LTA (tk- aprt-) mouse cells were cotransformed with pR plasmid DNA and ptk1 plasmid as selectable marker. Tk+ transformants were analyzed for their UV survival and for the presence of pR DNA sequences by blot-hybridization. The results show a correlation between the enhanced UV survival and presence of pR DNA sequences in cotransformed LTA mouse cells.