  Gamma-ray emission from cocoons of young radio galaxies is predicted.
Considering the process of adiabatic injection of the shock dissipation energy
and mass of the relativistic jet into the cocoon, we find that the thermal
electron temperature of the cocoon is typically predicted to be of the order of
$\sim$ MeV, and is determined only by the bulk Lorentz factor of the jet.
Together with the time-dependent dynamics of the cocoon expansion, we find that
young cocoons can yield thermal Bremsstrahlung emissions at energies $\sim$MeV.
Hotter cocoons (i.e., GeV) for younger sources are also discussed.
