1. 1. Hirata. K. etal Phys. Rev. Lett. 58, 1490-1493 (1987). 2. Biota, R.M. etal. Phys. Rev. Lett. 58, 1494-1496 (1987). 3. Colgate, S.A. & White, R.H. Astrophys. J. 143, 626 (1966). 4. De Rujula, A. Phys. Lett. 193B, 514-524 (1987). 5. Bahcall, J.N., Piran, T., Press, W.H. & Sperge!, D.N. Nature 327 682-685 (1987). 6. Eilis, J. & Olive, K.O. Phys. Lett. 193B, 525-530 (1987). 7. Schramm, D.N. Proc. XllmeRecontrede Moriond (1987). 8. Miller, D.]. Nature 329, 394-395 (1987). 9. Friemann, J.A., Haber, H.E. & Freese, K. SLAC Pub. 4261(1987). 10. Dar, A. & Dado, S. Phys. Rev. Lett, (in the press). 11. Barbiellini, G. & Cocconi, G. Nature 329, 21 (1987). 12. Kolb, E. & Turner, M. Phys. Rev. D (in the press). 13. Zatsepin, G.I. JETP Lett'%, 205 (1968). 14. Kolb, E., Stebbins, A.J. & Turner, M.S. Phys. Rev. D35, 3590(1987). 15. Spergel, D.N. & Bahcall, J.N. Phys. Lett. B (in the press). 16. Abbot, L.F., de Rujula, A. & Walker, T.P. Nucl. Phys. B (in the press).