The data presented here show for the first time that the protein known as “ribonuclease (RNase) inhibitor” (RI or RNH1) is present not only in the cell cytosol, but also in mitochondria, the central organelles in cell redox homeostasis. This finding directly correlates with the reported ability of RI to protect the cell from oxidative stress, with its sensitivity to oxidation and reactivity as a reactive oxygen species scavenger. While this study was carried out we also surprisingly discovered the presence of RI in the cell nucleus. We deem that these data open new views in the investigation on the cellular role(s) of the RI. STRUCTURED SUMMARY: RIphysically interacts with ATP synthase sub Alpha mitochondrial, Stress 70 Protein, Mitochondrial, Calcium-binding mitochondrial carrier protein Aralar 2ADP/ATP Translocase 2Elongation factor Tu and Cytochrome b-c1 complex Sub 2 by anti bait coimmunoprecipitation(View interaction) PARP and RIcolocalize by cosedimentation(View interaction)