Mutagenic abasic (AP) sites are generated directly by DNA-damaging agents or by DNA glycosylases acting in base excision repair. AP sites are corrected via incision by AP endonucleases, removal of deoxyribose 5-phosphate, repair synthesis, and ligation. Mammalian DNA polymerase beta (Polbeta) carries out most base excision repair synthesis and also can excise deoxyribose 5-phosphate after AP endonuclease incision. Yeast two-hybrid analysis now indicates protein-protein contact between Polbeta and human AP endonuclease (Ape protein). In vitro, binding of Ape protein to uncleaved AP sites loads Polbeta into a ternary complex with Ape and the AP-DNA. After incision by Ape, only Polbeta exhibits stable DNA binding. Kinetic experiments indicated that Ape accelerates the excision of 5'-terminal deoxyribose 5-phosphate by Polbeta. Thus, the two central players of the base excision repair pathway are coordinated in sequential reactions.