Mixed beams of ionizing radiation in our environment originate from space, the bedrock and our own houses. Radiotherapy patients treated with boron neutron capture therapy or with high energy photons are also exposed to mixed beams of gamma radiation and neutrons. Earlier investigations have reported additivity as well as synergism (a greater than additive response) when combining radiations of different linear energy transfer. However, the outcome seemed to be dependent on the experimental setup, especially the order of irradiation and the temperature at exposure. A unique facility allowing simultaneously exposure of cells to X-rays and 241Am alpha particles at 37 ºC was constructed and characterized at the Stockholm University (Paper I). To investigate the cytogenetic response to mixed beam irradiation (graded doses of alpha particles, X-rays or a mixture of both) several different cell types were utilized. AA8 Chinese Hamster Ovary cells were analyzed for clonogenic survival (Paper I), human peripheral blood lymphocytes were analyzed for micronuclei and chromosomal aberrations (Paper II and Paper III respectively) and VH10 normal human fibroblasts were scored for gamma-H2AX foci (Paper IV). For clonogenic survival, mixed beam results were additive, while a significant synergistic effect was observed for micronuclei and chromosomal aberrations. The micronuclei dose responses were linear, and a significant synergistic effect was present at all investigated doses. From the analysis of micronuclei distributions we speculated that the synergistic effect was due to an impaired repair of X-ray induced DNA damage, a conclusion that was supported by chromosomal aberration results. Gamma-H2AX foci dose responses were additive 1 h after exposure, but the kinetics indicated that the presence of low LET-induced damage engages the DNA repair machinery, leading to a delayed repair of the more complex DNA damage induced by alpha particles. These conclusions are not necessary contradictory since fast repair does not necessarily equal correct repair. Taken together, the observed synergistic effects indicate that the risks of stochastic effects from mixed beam exposure may be higher than expected from adding the individual dose components. / <p>At the time of the doctoral defence the following papers were unpublished and had a status as follows: Paper nr 3: Manuscript; Paper nr 4: Manuscript.</p> / DNA damage and repair in cells exposed to mixed beams of radiation