These experiments are a continuation of our work describing the effect of H2O2 and O2- on DNA strand breaks, NAD pools and poly(ADP-ribose) synthesis in C3H10T1/2 cells (Lautier et al. (1990) Biochem. Cell Biol. 68, 602-608). The current experiments were carried out firstly to evaluate the polymer synthesis in C3H10T1/2 cells exposed to benzamide, oxygen radicals and hyperthermia. Secondly, using four different protocols for the time of addition and removal of benzamide, the lowest benzamide levels shown to inhibit polymer synthesis were used to study the effect on plating efficiency and colony-forming ability of cells exposed to H2O2 and O2(-). Plating efficiency and colony-forming ability were affected by the active oxygen-species-generating system xanthine-xanthine oxidase and 100 microM benzamide. With higher levels of benzamide, this effect disappeared, and 0.5 to 1 mM benzamide were actually protective against the effects of xanthine-xanthine oxidase, suggesting the involvement of other processes in addition to poly(ADP-ribosyl)ation in response to oxygen radical damage.