  We present arguments in favor of the presence of free neutrons in plasmas
generated by compact astrophysical objects and find conditions necessary for
the formation of the neutron component. The broad range of phenomena caused by
neutrons includes both dynamical (sources' variability, transition of fireballs
to the two-flow regime) and kinetic (fission of helium nuclei by neutrons,
electromagnetic cascade, emission in annihilation and nuclear lines, neutrino
losses) effects. The presented theory can be applied to internal regions of
accretion disks, jets in microquasars, and gamma-ray burst (GRB) fireballs.
