Abstract
Within the framework of standard big-bang cosmology and the standard electroweak model, it is shown that νν¯ annihilation on the Z resonance is the only cosmic-ray process sensitive to relic neutrinos. For massive (mν103 eV) neutrinos originating from a z3 red-shifted source, a 15% to 50% absorption dip is predicted at Eν1011 GeV.