WR-2721 was investigated for its protective effect against acute and late damage produced by irradiation of the esophagus, small intestine, and colon of mice. The microcolony assay was used to measure the acute response of the small intestine and the colon, and an LD50 assay (within the 28- to 42-day time range) was used to measure acute esophageal damage. A dose of WR-2721 at 400 mg/kg, injected 30 min prior to irradiation, resulted in a protection factor (PF) of 1.6 against radiation damage in these three regions of the gastrointestinal tract. Lethality and histology scores were applied to determine late radiation damage to the rectum, at times ranging from 3 to 15 months after irradiation. Deaths occurred after doses of 20 Gy and above throughout the postirradiation period. WR-2721 increased the survival of mice; the PF calculated from the LD50 values was 1.5. PFs of animal survival did not vary during the observation period. Histological studies showed evidence of ulceration, fibrosis, and vascular changes as late radiation damage. WR-2721 protected against radiation-induced histological damage with a PF of 1.3. There was no qualitative difference between the types of histological damage observed in the group undergoing only irradiation and the group treated with WR-2721. Biochemical measurements of fibrosis by hydroxyproline determination of collagen 16 months after irradiation showed an increase in collagen per milligram wet weight of rectal tissue in all irradiated groups, but no increase in the amount of collagen per 5 mm segment of the rectum. Thus it appears that the apparent fibrosis is a result of atrophy rather than collagen accumulation. We conclude that WR-2721 is indeed effective at protection against late damage from large single doses of radiation to the rectum as measured histologically and also improves the long-term survival of the mice, although the target cells for this damage are not known.