A rhodium compound, Rh(NH3)3Cl3, does not sensitize the spores of Bacillus megaterium to X-rays. However, it is a very effective sensitizer of vegetative cells of Staphylococcus aureus, raising the sensitivity four times in O2 and over 100 times in anoxia. The inhibition by oxygen of the sensitizing action of Rh(III), which operates over a wide range of [O2], is noteworthy. These experiments were performed in saline-phosphate buffer using 50 kVp X-rays. The results are discussed in terms of the known radiation chemistry of this compound.