We have investigated the role of poly(adenosine diphosphate ribose) in DNA repair in human fibroblasts by observing the effects of 3-aminobenzamide (3AB), a specific inhibitor of poly(ADP-ribose) synthesis, on various aspects of DNA repair. After treatment of human fibroblasts with dimethyl sulfate (DMS), 3AB retarded the joining of strand breaks; unscheduled DNA synthesis was unaffected after low doses of DMS but was stimulated after high doses. 3AB also enhanced the cytotoxicity of DMS. After gamma irradiation there was a slight inhibition by 3AB of the rejoining of single-strand breaks but no effect on the rejoining of double-strand breaks, unscheduled DNA synthesis, DNA replicative synthesis, or cytotoxicity. There were no effects of 3AB on the repair of UV damage. On the basis of the different kinetics of the various steps of excision repair processes after different treatments of fibroblasts, our results are interpreted as evidence that the synthesis of poly(ADP-ribose) is involved in the ligation step of excision repair.