WR-2721 and its free-thiol metabolite WR-1065 have been characterized for their ability to protect mouse jejunal cells in vivo from the damaging effects of gamma rays with respect to both cytotoxicity and DNA single-strand break (SSB) induction. SSBs were measured both in the whole jejunal epithelium and in the proliferating crypt cells using an adaptation of the alkaline elution methodology. Protection factors (PFs) were also obtained using the microcolony assay for jejunal crypts. In mice treated with WR-1065 (400 mg/kg) 15 or 30 min prior to irradiation, there was a slight but significant reduction in the initial number of SSBs both in the whole jejunum (PF of between 1.17 and 1.22) and in the proliferating crypt cells (PF of between 1.13 and 1.28). At a dose of 200 mg/kg, the PF for SSBs in the proliferating crypt cells was 1.12 +/- 0.07 while that for crypt-cell survival was approximately 2.0. In mice treated with WR-2721 (400 mg/kg) 15 min prior to irradiation, there was little effect on the initial number of SSBs induced both in the whole jejunum (PF of 1.07 +/- 0.11) and in the proliferating crypt cells (PF of 1.04 +/- 0.07). WR-2721 protected jejunum in the microcolony assay with a much greater PF of 1.8. For each drug the PF for SSBs was therefore always much lower than that indicated by the biological end point under identical conditions. Both drugs also retarded the rate of SSB rejoining in each population of cells. These data suggest that mechanisms such as free-radical scavenging by these drugs may contribute to but not completely explain their protective action. Comparison with data obtained previously with cultured CHO cells supports the idea that the action of these drugs at the DNA lesion level may not be dose-modifying, but may also result in a shift in the spectrum of lesions induced by the radiation.