Abstract: The cytotoxic effects of bleomycin A₅ on S₁₈₀ both in vitro and in rive were synergistically enhanced by 3-aminobenzamide, a potent inhibitor of poly (ADP-ribose) polymerase, at concentrations at which 3-aminobenzamide alone did not show any cytotoxicity. This pontentiation was dependent on the concentration of 3-aminobenzamide with a dose-enhancement factor of about 4 in vitro. The inhibitory rate of bleomycin A₅ on S₁₈₀ in mice was increased from 41.4%, 37.4% and 66.0% to 60.1%, 72.4%, 68.3% and 77.8%, respectively in four separate experimentswhen used in combination with 3-aminobenzamide. The dose-effect relationship in vivo was also investigated.