The cross sections for 21Na production induced by 10-, 12-, 14-, and 16-MeV protons on natural Mg targets have been measured as 0.21¿0.08, 17.0¿2.5, 45.2¿6.8, and 44.0¿6.6 mb, respectively. These values imply that just below the surface the 22Ne/21Ne ratios found in extraterrestrial samples would be lowered by an excess of low-energy protons. Unfortunately, the lowering will in most cases be masked by other effects.