[en] Hamster and HeLa cells containing boron-10 at different concentrations were irradiated by a thermal neutron beam from a reactor. The survival curves were calculated according to the Katz and Sharma theory of track structure for heavy charged particles. The thickness of cell specimens irradiated was taken to be 0.02 cm to enable the first collision dose to be used. The boron-10 concentrations were 0, 2,5, 10, 20, 40 and 60 μg per g of tissue. For comparison with the experiments of Davis et al. the effect of fast neutrons was taken into account. Values for relative biological effectiveness (RBE) are given for different boron-10 concentrations and various surviving fractions. Isosurvival dose curves are defined and drawn which shows the relation between neutron fluences and absorbed dose for different boron-10 concentrations. The RBE values increase with decreasing dose and change only slightly with increasing boron-10 concentration for an equal surviving fraction. Some differences were found between the calculated results for HeLa cells in the thin layer and the experimental data for the cells in a monolayer. The results of the calculations are discussed. (author)