[en] The effects of different concentrations of the electron-affinic compound p-hydroxybenzoic acid (PHBA) on the radiation sensitivity of oxic and anoxic buffered suspensions of Staphylococcus aureus were examined. Increasing concentrations of PHBA from 10^-6 to 5 x 10^-5M in anoxic bacterial suspensions produced no change in the response characteristic of bacteria irradiated with gamma rays in anoxic buffer alone. However further increases in PHBA concentration to 6 x 10^-3M caused a marked increase in radiation sensitivity, with the maximal response very close to that for suspensions irradiated in the presence of oxygen. The lack of sensitizing action of PHBA in the presence of oxygen suggests that PHBA operates within the ' 0₂ effect '. The results of the study show that PHBA acts as an efficient hypoxic radiosensitizer when tested against Staphylococcus aureus. (UK)