An investigation of the factors affecting the biological effectiveness of neutron beams suitable for Boron Neutron Capture Therapy (BNCT) has been carried out. The primary experimental work described in this thesis concerns the degree of interaction, if any, between biological damage caused by low LET radiation and that caused by high LET radiation. The second area investigated concerns the biological impact of delivering a BNCT irradiation at differing dose rates. In mixed photon alpha particle irradiations, no synergistic effect was observed above the response from the separate components. Maximum alpha particle doses delivered were 2.54 Gy. In mixed X-ray and alpha particle exposures, no synergy effect was seen with 2.54 Gy of alpha particles delivered to the cells. At the 3.18Gy alpha particle dose level significantly lower cell survival was observed than would be predicted from survival in single fields. Dose rate experiments were carried out in the Massachusetts Institute of Technology (MIT) Fission Converter neutron Beam (FCB). Cells loaded with boric acid were exposed at dose rates differing by a factor of approximately 15. A dose rate effect was observed at both of the irradiation depths used, although this was only clearly significant at 50 mm treatment depth.