Detergent-lysed BS-C-1, HeLa, and mouse L cells incorporate ADP-ribose from NAD+ into two classes of macromolecules. Metabolically stable products, which appear to be a variety of proteins to which are attached one or a few ADP-ribose residues, predominate when the cellular DNA remains intact. In addition, ghost cells have a potentially much greater capacity to synthesize poly(ADP-ribose), which is completely dependent upon the introduction of strand breaks into their DNA. The initial rate of poly(ADP-ribose) synthesis increases linearly with prior x-ray dose or with the concentration of endonuclease added and, once synthesized, the polymer is rapidly degraded with a half-life of 10 min or less. It appears that sites on the DNA capable of supporting a certain amount of poly(ADP-ribose) synthesis are created as a result of x-irradiation or nucleolytic cleavage and are rapidly eliminated, or “repaired,” during subsequent incubation. The sites accumulate if cells are irradiated at 0 degree C; further incubation of the lysed cells with NAD+ at 35 degrees C results in both a burst of poly(ADP-ribose) synthesis and the elimination of the sites. NAD+ enhances the elimination of x-ray-induced sites. Thus, the synthesis of poly(ADP-ribose) may be required for the repair of DNA strand breaks.