The Monte-Carlo technique was used to perform quantitative microdosimetric model calculations of cell survival after boron neutron capture irradiations in vitro. The high energy 7Li and alpha-particles resulting from the neutron capture reaction 10B (n,alpha)7Li are of short range and are highly damaging to cells. The biophysical model of the Monte-Carlo calculations is based on the track structure of these a-particles and 7Li-ions and the x-ray sensitivity of the irradiated cells. The biological effect of these particles can be determined if the lethal effect of local doses deposited in very small fractional volumes of the cell nucleus is known. This lethal effect can be deduced from experimental data of cell survival after x-ray irradiation assuming a Poisson distribution for lethal events. The input data used in a PC-based computer program are the radial dose distribution inside the track of the released particles, cell survival after x-ray irradiation, geometry of the tumor cells, subcellular 10B concentration, and thermal neutron fluence. The basic concept of this Monte-Carlo computer model is demonstrated. Validations of computer calculations are presented by comparing them with experimental data on cell survival.