  We quantify the limits on quark-antiquark mass differences imposed by the
neutral kaon mass system. In particular, we find that an upper limit to the
mass difference of 10^{-3} eV exists if mass differences across quark flavors
are uncorrelated. In the upcoming antihydrogen experiments this limit on quark
mass difference would allow a measurement of electron-positron mass difference
up to a relative precision level of 10^{-15}.
