Diploid XP cells were exposed to demecolcine for three days and then released from this exposure. Tetraploid XP cells were successfully established 60 d after drug addition. The doubling time of the tetraploid cells was 18 h, identical to that of diploid cells. The tetraploid cells had 92 chromosomes and double the DNA content of the parent diploid cells. The cell volume of tetraploid cells was about twice that of the diploid cells. The duration of the G₁, S and G₂/M phases was almost the same as for diploid cells. It was concluded that the established tetraploid XP cells have cellular characteristics similar to those of diploid XP cells. The rate of survival after UV irradiation was lower in tetraploid XP cells than in diploid XP cells.