The risk to human health of exposure to low-level radiation is not precisely known yet. One way of studying this is to carry out in vitro biological experiments with cell cultures and to extend the conclusions to biological models. To relate the macroscopically deteminable 'low dose' to the damage of cells caused by a certain type of ionising particle is nearly impossible. therefore the number of hits and the imparted energy are the significant quantities. They can be estimated by particle transport calculations and by direct measurements. The effect of low dose was investigated in radio-adaptation experiments when mono-layers of different unsynchronised cell cultures were irradiated by neutrons produced in the filtered beam of the Budapest Research Reactor (BRR). The energy deposition was investigated by replacing the mono-layers with etched track detectors of the CR-39 type.