  Astrophysical gamma-ray sources come in a variety of sizes and
magnetizations. We deduce general conditions under which gamma-ray spectra from
such sources would be significantly affected by axion-photon mixing. We show
that, depending on strength and coherence of the magnetic field, axion
couplings down to ~ 1/(10**13 GeV) can give rise to significant axion-photon
conversions in the environment of accreting massive black holes. Resonances can
occur between the axion mass term and the plasma frequency term as well as
between the plasma frequency term and the vacuum Cotton-Mouton shift. Both
resonances and non-resonant transitions could induce detectable features or
even strong suppressions in finite energy intervals of gamma-ray spectra from
active galactic nuclei. Such effects can occur at keV to TeV energies for
couplings that are currently allowed by all experimental constraints.
