An electrophoretic system was established that resolves poly(adenosine diphosphate ribose), enzymatically synthesized polymer from NAD+, by size difference of one residue on polyacrylamide gel. The existence of a polymer of at least 65 residues was demonstrated by band counting in this system. The polymer showed a heterogeneous size distribution on the electrophoregram, and the molecular weight of the largest polymer was deduced to be more than 4.5 X 10(5) daltons. The discrepancy between the size, estimated by electrophoresis, and the chain length, determined by the ratio of total radioactivity to that derived from the terminus, suggests that the polymer has a branched structure.