  We propose that axion-like particles (ALPs) with a two-photon vertex,
consistent with all astrophysical and laboratory bounds, may lead to a
detectable signature in the spectra of high-energy gamma ray sources. This
occurs as a result of gamma rays being converted into ALPs in the magnetic
fields of efficient astrophysical accelerators according to the "Hillas
criterion", such as jets of active galactic nuclei or hot spots of radio
galaxies. The discovery of such an effect is possible by GLAST in the 1-100 GeV
range and by ground based gamma ray telescopes in the TeV range.
