The addition of bovine superoxide dismutase to Brucella broth or Brucellar agar greatly enhanced the oxygen tolerance ofCampylobacter fetussubsp.jejunistrain H840 (ATCC 29428). Catalase also enhanced oxygen tolerance, but to a lesser extent. These enzymes must act externally to the bacteria. All of the diverse compounds which enhance oxygen tolerance ofC.fetus, including nor-epinephrine and a combination of ferrous sulfate, sodium metabisulfite, and sodium pyruvate, share the ability to quench either superoxide anions or hydrogen peroxide. On the basis of these and other data, we propose thatC.fetusis more sensitive to exogenous superoxide anions and hydrogen peroxide than are aerotolerant bacteria, despite the occurrence of superoxide dismutase and catalase activities inC.fetus. Compounds that enhance oxygen tolerance inC.fetusappear to act by quenching superoxide anions and hydrogen peroxide which occur spontaneously in the culture medium.