  We discuss polarizing a proton beam in a storage ring, either by selective
removal or by spin flip of the stored ions. Prompted by recent, conflicting
calculations, we have carried out a measurement of the spin flip cross section
in low-energy electron-proton scattering. The experiment uses the cooling
electron beam at COSY as an electron target. The measured cross sections are
too small for making spin flip a viable tool in polarizing a stored beam. This
invalidates a recent proposal to use co-moving polarized positrons to polarize
a stored antiproton beam.
