Exogenous prostaglandins (PGs) have been shown to protect gastrointestinal mucosa, liver, and pancreas from several injurious agents, including the PG inhibitor, indomethacin. Previous studies from this laboratory showed exogenous administration of 16,16-dimethyl (dm) PGE₂ also protected mouse intestinal stem cells from radiation injury. The present study extended that observation and demonstrated that PGs given to B6D2F1 mice 1 hr before irradiation increased the shoulder of the intestinal clonogenic cell survival curve. The D₀ increased from 1.10 + 0.09 to 1.58 + 0.10 Gy. PGs increased the LD50/6 from 16.3 + 0.41 (95% confidence limits) in controls to 20.25 + 0.55 Gy. The 16,16-dm PGE₂ increased the hematopoietic CFU-S survival in a qualitatively similar way; the extrapolation number (n) was increased from 1.03 (0.89-1.20) to 1.40 (1.27-1.54) and the D₀ increased from 0.92 (0.87-0.98) to 1.14 (1.10-1.19) Gy. A large number of human tumors secrete a variety of PGs. Our results suggest that those tumors may be, in part, protected from radiation injury.