  We discuss the role of thermal cyclo-synchrotron emission in a magnetically
dominated corona above an accretion disk in an active galaxy or a galactic
black hole candidate. The dissipation occurs in localized active regions around
the central black hole. Cyclo-synchrotron radiation is found to be an important
process. In the case of galactic black hole candidates, its emission can
dominate the inverse Compton scattering of the soft photon field produced by
the disc by thermal electrons. We discuss observational predictions and the
detectability of cyclo-synchrotron radiation both for these sources and
radio-quiet active galactic nuclei.
