[en] The yield of inactivation of radioresistant strains of E. coli K-12 (wild type and uvr A mutant) suspended in phosphate-buffered saline (PBS) is higher in suspensions irradiated under N₂O-bubbling than in suspensions irradiated under N₂-bubbling. This sensitization is largely a result of OH-induced toxic products formed in N₂O-saturated PBS. Special emphasis is placed on establishing the conditions under which extracellular toxic products with lifetimes exceeding about 20 sec are formed. It is concluded that chlorine atoms are precursors for these toxic products and that N₂O and phosphate potentiate their formation