The cDNA encoding the carboxyl-terminal 40-kDa domain of human poly(ADP-ribose) polymerase was inserted into an expression vector. The recombinant protein was overproduced in Escherichia coli, and purified to homogeneity. The 40-kDa domain had the same affinity (Km) for NAD+ as the full-length enzyme, expressed abortive NAD+ glycohydrolase activity, catalyzed the initiation, elongation, and branching of ADP-ribose polymers, but exhibited no DNA dependence. Its specific activity was approximately 500-fold lower than that of the whole enzyme activated by DNA strand breaks. Surprisingly, the carboxyl-terminal 40-kDa domain exhibited the processive mode of polymer attachment typical of full-length poly(ADP-ribose) polymerase and was able to modify histones H1 and H2B. Finally, the polymer sizes formed by the 40-kDa domain were influenced by histone H1.