The biochemical role of polyADP-ribosylation on the internucleosomal DNA fragmentation associated with apoptosis was investigated in HL60 human premyelocytic leukemia cells. An increase in the activity of poly(ADP-ribose) polymerase preceded DNA fragmentation during the DNA damaging agent induced apoptotic process. During non-DNA damaging agent induced apoptotic process, however, the activity of poly(ADP-ribose)polymerase increased after DNA fragmentation. UV and A23187 increased polyADP-ribosylation of nuclear protein, particularly histon H1. Inhibitor of poly(ADP-ribose)polymerase, 3-aminobenzamide prevented polyADP- ribosylation of histon H1 in both UV and A23187 induced apoptosis. 3-aminobenzamide prevented both internucleosomal DNA fragmentation and apoptotic process, suggesting a requirement of polyADP-ribosylation for DNA fragmentation in DNA damaging agent induced apoptosis. 3-aminobenzamide did not prevent neither DNA fragmentation or apoptotic process in non-DNA damaging agent induced apoptosis. These results suggest that apoptosis- inducing mechanisms of various agents were diverse, but, DNA damaging agent have common step including PARP activation and histon H1 polyADP- ribosylation along their apoptotic pathways.