We have found that human cells respond to treatment with an alkylating carcinogen by producing a specific, highly conserved array of poly(ADP-ribose) molecules. Using in situ radiolabeling and boronate affinity chromatography, we have been able to isolate ADP-ribose polymers from living cells and to quantify individual molecular size classes on high resolution polyacrylamide gels. Despite carcinogen-induced changes in poly(ADP-ribose) turnover, the relative frequency of linear polymers was strictly conserved. However, the abundance of branched polymers with high affinity for histones increased 2.6-fold. These polymers were degraded more slowly than linear polymers. Our results indicate that the poly(ADP-ribose) molecules involved in the shuttling of histones on DNA in vitro also play a role in the repair of DNA damage in vivo.