Nitrosophenylhydroxylamine-ammonium (cupferron), potassium cyanide, sodium azide, ethylenediaminetetraacetate (EDTA), α,α'-dipyridyl, and o-phenanthroline were tested (1) for their ability to enhance the frequencies of chromosomal aberrations produced by x-rays in the root tip cells of the broad bean, Vicia faba, and (2) for their ability to inhibit oxygen consumption of excised roots of the same plant. In all cases a close correlation was found between the inhibitory effect on respiration and the enhancement of the sensitivity to x-rays at low oxygen pressures. EDTA, dipyridyl, and o-phenanthroline did not affect respiration to any greater extent, and they were without influence on the radiosensitivity. Cyanide, azide, and cupferron, which strongly inhibited respiration, also increased the frequencies of chromosome aberrations produced by x-rays at low oxygen pressures.

The relation between oxygen concentration and radiosensitivity was determined both in the presence and the absence of the respiratory inhibitor cupferron. When cupferron was present, the radiosensitivity was influenced by oxygen concentrations 30 times lower than those effective in the absence of the inhibitor.

In an atmosphere of pure oxygen, an increase of radiosensitivity of about 20 per cent was obtained with cupferron, EDTA, and potassium cyanide.

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