A beam consisting of mainly 24 keV neutrons has been constructed for radiobiological studies of evaluate the potential of these particles for treating deep tumours by the boron capture reaction. The induction of chromosomal aberrations in human lymphocytes in vitro was examined and a linear dose effect with a relative biological effectiveness similar to fission neutrons was obtained. For samples placed at depths in a plastic phantom the aberration yields declined with depth at a rate matching the fall in the sum of dose due to proton recoils and neutron capture in nitrogen 14. The presence of boron 10 at 30 μg ml-1 did not affect the aberration yield. By using the mixed sample method, the probability of interphase death or mitotic delay in cells crossed by an α particle or lithium-7 ion produced in the boron capture reaction was shown to be close to 1.0. Thus these cells are prevented from coming to mitosis in culture. The implications for boron capture therapy are that this filtered beam has a 'high LET' effect which could lead to poor normal tissue sparing. However there may be a significant therapeutic advantage due to a high probability of killing tumour cells that have incorporated boron 10.