A cDNA for human poly(ADP-ribose) polymerase was inserted into a plasmid, transfected and expressed in E. coli. A lysate of the E. coli cells containing the expression plasmid reacted with antibody against human poly(ADP-ribose) polymerase and synthesized poly(ADP-ribose). The partially purified poly(ADP-ribose) polymerase expressed in E. coli had the same molecular weight and enzymological properties as human placental poly(ADP-ribose) polymerase, including affinity for NAD, turnover number and DNA-dependency for activity. This expression system should be useful for structure-function analysis of poly(ADP-ribose) polymerase.