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The RBE for damage to the small intestine should be determined prior to treatment of cancer in the abdomen and pelvis with fast neutrons. It is particularly important to make certain that the RBE for late damage is no greater than the RBE for acute injury to this organ. In the present study the LD 50 5 day following whole-body irradiation was used to measure the sentivity of the intestinal tract to acute injury. The LD 50 90 day, following irradiation of a 4 cm segment of small intestine, was used to assess late damage to this structure. Relative to X-radiation, the fast neutron RBE for late damage (1·9) did not differ significantly from the fast neutron RBE for acute injury (1·8). The mechanism of death following partial intestine irradiation with neutrons or X-radiation was associated with bowel obstruction. A neutron dose of 1·2 krad delivered to only a small segment of the small intestine produced 50% mortality within 90 days.