Activity of ornithine decarboxylase (ODC) in Cockayne syndrome (CS) cells which were characterized by a delay of the recovery of DNA synthesis after 254 nm ultraviolet light (UV) radiation was extremely inhibited by UV, followed by a progressive decrease of putrescine. When putrescine was added in the cultures, CS cells showed a rapid recovery in DNA synthesis and also an expression of UV-resistance as seen in normal cells. Thus, ODC induction system of CS cells is highly sensitive to UV. Considering current hot spots in DNA repair, our results probably reflect a defect of preferentially rapid repair in ODC gene of CS cells.