  We propose an experimental test of the gravitational interaction with
antimatter by measuring the branching fraction of the CP~violating decay
$K_\mathrm{L} \to \pi^{+} \pi^{-}$ in space. We show that at the altitude of
the International Space Station, gravitational effects may change the level of
CP~violation such that a 5$\sigma$ discrimination may be obtained by collecting
the $K_\mathrm{L}$ produced by the cosmic proton flux within a few years.
