In the bacterial spore system, both oxygen and p-nitroacetophenone (PNAP) function as radiation sensitizers. Previous work had shown that PNAP has a protective effect in$0.8\%\ {\rm O}_{2}$. These results, obtained in phosphate buffer (pH 7.0), have now been compared with results from water-suspension experiments. In water, mixtures of PNAP and oxygen show the same response as$0.8\%\ {\rm O}_{2}$ alone; i.e., PNAP does not protect. Although the mechanisms are not understood, the previously observed protection by PNAP in$0.8\%\ {\rm O}_{2}$ must be designated as buffer dependent. Therefore, spore studies to examine chemical sensitizing processes of PNAP and O2 should be in water, not phosphate buffer.

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