The relationship between treatment with 3-methoxy-benzamide (MBA), a potent inhibitor of ADP-ribosylation reactions, and the response of C3H10T1/2 cells to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) has been examined. Quiescent cells effected potentially lethal damage repair (PLDR) over a 48-h period following MNNG and the repair was coincident with the removal of DNA strand breaks. MBA had no effect on PLDR but was very co-cytotoxic with MNNG in dividing cells. The presence of MBA caused the appearance of an additional number of DNA strand breaks following MNNG in both quiescent and dividing cells. These results suggest that ADP-ribosylation is required for normal cell cycle progression following DNA damage in dividing cells.