Small admixtures in water, e.g. of metal ions, often act as cell growth regulators. Here we report that enrichment of deuterium content in water, normally found at 8 mM concentration, two-three folds increases cell proliferation and lowers the oxidative stress level as well. Acting as an anti-oxidant, deuterium-enriched water prevents the toxic effect of such oxidative agents as hydrogen peroxide and auranofin. This action is opposite to that of deuterium depletion that is known to suppress cell growth and induce oxidative stress in mitochondria. We thus hypothesize that deuterium may be a natural cell growth regulator that controls mitochondrial oxidation-reduction balance. Since growth acceleration is reduced approximately by half by addition to water a minute amount (0.15%) of 18O isotope, at least part of the deuterium effect on cell growth can be explained by the isotopic resonance phenomenon.