[en] Male CD₂F₁/HAP mice were exposed to similar doses from Cs-137 and Cf-252 linear source arrays using a ferris wheel irradiator. The Cs-137 γ-ray dose rate was 87.2 cGy/hr as determined by TLD dosimetry. A paired ionization chamber measurement technique was utilized to evaluate the Cf-252 dose rate; i.e. 14.4 cGy/hr neutron and 6.5 cGy/hr gamma rays (n/γ ratio = 2.22). Jejunal crypt scoring criteria followed those established by Withers and Elkind. These experiments show that RBE in small bowel was high using microcolony assay. Analysis of crypt survival curves yielded a RBE for Cf-252 neutrons of approximately 6.0 for small bowel in agreement with data reported by Withers and Elkind. (Auth.)