The p53 tumor suppressor protein plays an essential role in cellular integrity and inactivation of the TP53 gene by mutation is the most frequent alteration in human cancer. As loss of p53 function is associated with increased genetic instability, it is important in genotoxicity testing to explore the role of p53 competency.In vitromodel systems for genotoxicity testing are sometimes prone to misleading positive results; some of this loss of predictivity may be caused by p53 inactivation in some cell models. To explore whether impaired p53 function plays a role in mutation sensitivity, TK6 cells (p53 competent) and NH32 cells (p53 deficient) were treated with two known genotoxicants, mitomycin C (MMC) and cytosine arabinoside (araC). Chromosomal damage was assessed in the low dose region by an automated micronucleus system and p53 activity was investigated by gene and protein expression analysis. Cell cycle progression studies were also assessed. Low levels of micronucleus and p53 induction were observed in TK6 cells treated with MMC. On the other hand, higher levels of micronucleus and p53 induction were shown in TK6 cells treated with araC and a G1/S arrest was observed after araC treatment. p53 deficient NH32 cells showed an increased sensitivity of micronucleus (MN) induction after araC treatment compared with TK6 cells and less of an active G1/S phase checkpoint. Thus, impaired p53 function sensitizes cells to genotoxicants and plays a central role in the DNA damage response. This data has clear importance for safety assessment of genotoxicity and shows how crucial p53 competence is.