Thalassaemia is a group of genetic diseases where haemoglobin synthesis is impaired. This chronic anaemia leads to increased dietary iron absorption, which develops into iron overload pathology. Treatment through regular transfusions increases oxygen capacity but also provides iron through the red cells' haemoglobin. An essential treatment, in parallel with transfusions, is the use of chelating agents to remove the excess iron deposited in tissues. These deposits are found in the liver, spleen, heart, and pancreas and are associated with cardiac failure and diabetes. The deposits in these tissues of patients have been isolated as haemosiderin. Thalassaemia patients are particularly at risk of free radical induced damage. Thus, the present study has investigated, as a model system, human cells in vitro in the Comet assay in the presence of free radicals. This assay measures DNA damage, particularly DNA strand breakage. The effects of iron overload on cells oxidatively stressed with hydrogen peroxide (H2O2) have been determined as well as the effect of the chelating agent, deferoxamine. Iron overload was simulated with ferric (FeCl3) and ferrous chloride (FeCl2), ferrous sulphate (FeSO4) and haemosiderins. Both human lymphocytes from a male and a female donor and human adenocarcinoma colonic cells showed an increase in DNA damage in the Comet assay after treatment with H2O2. Ferric chloride produced an increase in DNA damage in human colonic cells, but little or no damage in human lymphocytes. Ferrous chloride also produced weak DNA damage in human lymphocytes, but ferrous sulphate produced a dose-related response. Deferoxamine produced no DNA damage. When H2O2 was combined with FeCl3, FeCl2, or FeSO4, the DNA damage produced was as least as great as or slightly greater than with H2O2 alone. When deferoxamine was combined with H2O2 and FeSO4 there was a consistent decrease in response. There was little or no decrease in response when deferoxamine was combined with H2O2 and FeCl3 or FeCl2, but at high (100-300 μm) doses there were changes in the appearance of cellular DNA from Comet tails to dense centres surrounded by a diffuse area. This was probably as a consequence of chelation processes. Haemosiderin produced no damage. The three fractions of haemosiderin examined were of three different densities and from a Thai patient where the oxyhydroxide phase is the ferrihydrite. The colour change was similar to that for FeCl3, but the level of the ferric ion in the haemosiderin was possibly too low in the sample to produce a response. The next stage is to examine peripheral lymphocytes from thalassaemic patients, with and without chelation therapy, whose cells may be more sensitive to simulated iron overload and to lower levels of haemosiderin.