LAf1 mice exposed to sublethal doses of fast neutrons (290 to 580 rads) or to lethal doses of X-ray (800 r or 1000 to 1100 r) were followed until natural death in order to study long-term irradiation effects. The X-irradiated mice were protected against acute death by injection of hematopoietic cells or limb shielding. Gastrointestinal carcinomas were much more common in the neutron-irradiated than in the X-irradiated (800 r) mice, and a neutron RBE value of 2 to 3 for these tumors was indicated. Nephrosclerosis, on the other hand, was more common in the X-irradiated mice, with an apparent neutron RBE value close to unity for this lesion.