Abstract : This study is mainly concentrated on the effects of ionizing radiations on living cells. It contains three main parts: The first part describes the "cell damage and cell repair" processes that occur during and after irradiations, as well as the results obtained by Dionet et al. in the "Laboratoire de Physique Corpusculaire" which confirmed that human melanoma cells exhibit a certain hypersensitivity low doses and low dose rates irradiations with 14 MeV neutrons. A bibliographical research confirms that this same hypersensitivity was also observed by other authors. The second part describes the development of the GEANT4DNA toolkit which is a Monte- Carlo code that can follow protons (till energies as low as 100 eV) and electrons (till 15 eV) in liquid water. This code was used to describe recoil protons traces in the cell during 14 MeV neutrons irradiations. The third part describes the microdosimetric formalism and the theory of dual radiation action which was used in tour study to estimate the relative biological effect of 14 MeV neutrons on the melanoma cells. The obtained RBE was compared to the experimental results observed by Dionet et al.