V79 Chinese hamster cells were irradiated in the presence of different amounts of boric acid with thermal neutrons at the Medical Research Reactor at Brookhaven National Laboratory. From the linear dose-survival curves observed, a D0 value of 66 rad for the 10B(n, alpha) 7Li neutron capture reaction was obtained. No dependence of this value on the concentration of boric acid was found. Comparing this value to the D0 value of 150 rad obtained with 250 kVp X rays between 10 and 0.01% survival, an extrapolated RBE value of 2.3 was calculated. By irradiation of the same line of cells with cold neutrons at the Institut Laue - Langevin , a D0 value for the 14N(n,p)14C reaction of 77 rad was obtained, with a corresponding RBE value of 1.9. Comparison is made with previously published RBE values for the 10B(n, alpha) 7Li reaction.