Paraoxonase 1 (PON1) is an enzyme synthesised in the liver that is associated with High Density Lipoprotein (HDL) and increasingly with a number of human diseases, including cardiovascular disease and cancer. PON1 is (i) a lactonase, hydrolysing aliphatic and aromatic lactones, (ii) a phosphotriesterase, acting on certain organophosphates and (iii) a peroxidase that combats lipid oxidation. The aim of this study was to investigate the extent to which PON1 might impact on the levels of lactone-induced and oxidative DNA damage. Initially, the plasmid DNA (pDNA) nicking assay was used to show that of the 12 lactones and homocysteine (HC) examined, only alpha angelica lactone (AAL), Furanone (FUR) and HC caused DNA single strand breaks (SSB) under physiological conditions. Further studies indicated that AP sites were not involved suggesting that DNA-phosphotriesters were responsible. AAL-reacted pDNA bound the damage sensing protein, Atl1, and AAL, HC and HC thiolactone (HCTL) -reacted calf thymus DNA inhibited the activity of the DNA repair protein, MGMT, both indicating the presence of O6-alkylguanines in DNA, although this could not be confirmed using MALDI-ToF MS analyses of tryptic digests of MGMT incubated with lactone-reacted DNA. The inhibition of rPON1 by these lactones and HC was determined using paraoxon as a substrate and two groups were identified comprising lactones that caused reductions in PON1 activity of (i) ˃15% (e.g. HCTL, and AAL) and (ii) ˂10% (e.g. FUR, and HC). The pDNA nicking assay then showed that only AAL and FUR induced DNA single strand breaks. PON1 itself nicked pDNA, and bound to group 1 lactone-reacted pDNA by an unknown mechanism, both effects not previously reported, but, with the possible exception of AAL, did not increase the extent of plasmid nicking. The MTT, cell viability assay, indicated that all of the lactones with the exception of γ-BL (IC50 >12 mM) were to some degree toxic in HepG2 cells with AAL being the most toxic (IC50 1.0 ±0.03 mM). It was not possible to quantify PON1 activity in HepG2 cells and agents that are reported to change the expression levels of PON1 had no detectable impact on the toxicity of AAL, γ-VL, FUR or MBL, so any possible effect of PON1 could not be determined. The neutral Comet assay showed that AAL and HCTL generated the highest levels of DNA double strand breaks and DNA fragmentation in HepG2 cells, and that this effect was greatly enhanced for most of the lactones by the addition of rPON1. The impact of PON1 on oxidative stress was investigated using serum samples collected as part of a previous case-control study of lung cancer. Initially, an in-house developed ELISA assay to quantify 8-hydroxydeoxyguanosine (8-OHdG) levels had insufficient sensitivity and poor reproducibility. Hence 8OHdG levels were measured using kits along with PON1 levels and other serum parameters including HDL-C, LDL-C, TG and apoAI in serum from 112 patients with lung cancer and 249 patients without. No correlation was found between serum level of PON1 activity and level of 8-OHdG in patients with lung cancer, however a negative non-significant correlation was found between PON1 and 8-OHdG in control. The level of 8-OHdG was significantly higher in lung cancer patients than in controls and in the controls, the OGG1 wild type genotype correlated with reduced levels of 8-OHdG in serum. These studies showed that certain lactones, are toxic and DNA damaging and this can be increased by PON1, suggesting that any association between PON1 and human disease will be substrate dependent and may be PON1 genotype-dependent.