TBI at doses ≥2 Gy results in loss of neutrophils and severe neutropenia by 7 days postirradiation. Sham-irradiated animals did not show any significant changes in neutrophil counts, while 2 Gy irradiated animals showed a significant drop in neutrophils (ANOVA P value = 5.0 × 10^–5), but no instances of severe neutropenia (counts <500 µL^–1) were observed. 3 out of 6 animals that were exposed to 5.5 Gy, and 5 out of 7 animals exposed to 6.5 Gy developed severe neutropenia. This dose-dependent response is consistent with our previous findings (6). NHPs exposed to 9.8 or 10.7 Gy WTLI did not develop severe neutropenia by day 7 (25).