  The transfer of polarized radiation in magnetized and non-magnetized
relativistic plasmas is an area of research with numerous flaws and gaps. The
present paper is aimed at filling some gaps and eliminating the flaws. Starting
from a Trubnikov's linear response tensor for a vacuum wave with ${\bf
k}=\omega/c$ in thermal plasma, the analytic expression for the dielectric
tensor is found in the limit of high frequencies. The Faraday rotation and
Faraday conversion measures are computed in their first orders in the ratio of
the cyclotron frequency $\Omega_0$ to the observed frequency $\omega$. The
computed temperature dependencies of propagation effects bridge the known
non-relativistic and ultra-relativistic limiting formulas. The fitting
expressions are found for high temperatures, where the higher orders in
$\Omega_0/\omega$ cannot be neglected. The plasma eigenmodes are found to
become linearly polarized at much larger temperatures than thought before. The
results are applied to the diagnostics of the hot ISM, hot accretion flows, and
jets.
