A series of poly (ADP-ribose) synthesis inhibitors was evaluated for their ability to potentiate the cytotoxicity of bleomycin compared with their inhibition of poly(ADP-ribose) synthesis in L1210 cultured cells. Theophylline, nicotinamide, 3-aminobenzamide and thymidine inhibited 70 to 80% of poly(ADP-ribose) synthesis in L1210 cells. The degree of inhibition of poly(ADP-ribose) synthesis by these inhibitors corresponded in general with their potentiating ability of bleomycin cytotoxicity. Among these inhibitors, 3-aminobenzamide significantly potentiated the cytotoxic activity of bleomycin (3.6 fold), but it could not potentiate the cytotoxic activity of other antitumor agents, including nitrosourea and Cis-DDP, in L1210 cells in vitro. Treatment of Ehrlich ascites tumor bearing mice with bleomycin and 3-aminobenzamide produced a synergistic effect. 6-Aminonicotinamide also potentiated the antitumor activity of bleomycin against Ehrlich ascites tumor.