Aqueous suspensions of liveEscherichia Coli B (E. ColiB)bacteria cells were irradiated in the resting state under aerobic and anoxic conditions with 0.8 MeV electrons. Dried films of these suspensions were studied using infrared spectroscopy in the range from 4000 cm−1(2.5 Mm) to 700 cm−1(14.3 μm). The results show selective damage in that some molecular bonds are affected more than others. Changes observed include breakup of biopolymers, overall oxidation, nucleic acid base damage, and strand breaks in the DNA and RNA of cells. Oxygen is found to act as a powerful catalyst enhancing the amount of damage observed by a factor of 5 ± 2 over the damage observed under anoxic conditions. The minimum absorbed doses necessary to observe the onset of molecular alterations were about ten times higher than the D37dose for cell death in bacteria.