Dietary antioxidants are thought to be beneficial for human health. They can prevent damage to biomolecules such as DNA by removing free radicals and consequently prevent oxidative stress. However, several large scale intervention studies have found no beneficial effects or even harmful effects of antioxidant supplementation. More studies are therefore needed to sort out whether, how or when antioxidants improve health.Astaxanthin is a marine carotenoid synthesised by algae, which gives salmon, krill and other aquatic animals their red colour. It has been reported that astaxanthin has a particularly high antioxidant capacity. A few clinical studies have investigated the effects of astaxanthin supplementation on human health, with some promising results, but none of them has directly measured the effects on DNA damage. The aim of this project was to investigate whether astaxanthin can protect the DNA from damage in cell cultures and in vivo, and whether it has an effect on DNA repair in cultured cells using the comet assay. This technique is one of the standard methods for measuring DNA damage, and in this study it was used to measure DNA strand breaks and oxidised bases. In cell culture experiments, HeLa and Caco-2 cells were incubated with 0, 0,3, 1, 3 or 10 μM astaxanthin for 2 or 24 hours, and endogenous DNA damage was measured. Incubation with astaxanthin did not have any effect on the amount of DNA strand breaks, oxidised purines and oxidised pyrimidines. In similar experiments astaxanthin pre-incubated cells were treated with H2O2 to induce strand breaks or the photosensitizer Ro 19-8022 plus light to induce DNA oxidation. In these experiments astaxanthin did not protect against the DNA damage induced by any of the agents. DNA repair capacity of strand breaks was measured in astaxanthin pre-incubated HeLa cells by inducing strand breaks and then monitoring the cells’ ability to repair their DNA by measuring the remaining DNA damage at different time intervals. Pre-incubation of the cells with 3 or 10 μM astaxanthin did not affect the DNA repair capacity.To investigate the effects of astaxanthin supplementation on humans, a randomised double-blind intervention study with a cross over design was executed. Twenty-three healthy, non-smoking men and women consumed capsules with a total of 20 mg of astaxanthin from an algal extract and/or placebo capsules each day for 3 weeks. Blood samples were collected at study start and before and after each intervention period. DNA damage was measured in peripheral blood lymphocytes with the comet assay. Supplementation did not affect the amount of endogenous DNA damage (strand breaks, oxidised purines and pyrimidines), nor the DNA susceptibility to DNA damage induced by treatment with H2O2.The lack of effect from astaxanthin was clear and consistent, not even a hint of a trend was observed. These results are in contrast to results from other in vitro experiments demonstrating a potent antioxidant effect of astaxanthin, and a few cell culture and human studies demonstrating protection against oxidative stress. Further investigations should be to measure astaxanthin levels in plasma stored from the human intervention trial. Peripheral blood lymphocytes were also stored to measure DNA damage in a randomised fashion or in batches, and to measure DNA repair.