The steady-state radiation loss of a homogeneous and isothermal spherical plasma due to an optically thick spectral line is calculated under the assumption that all other transitions of the considered atoms remain optically thin. The line profile is approximated by a rectangular profile and by a Doppler profile, respectively. The energy losses found can be described in almost all cases of practical interest by a volume radiation formula and are in general much smaller than the losses corresponding to thermal radiation. In the case of an optically thick plasma emitting radiation far from thermal equilibrium the spectral distribution of the intensity is double humped simulating a self absorption effect of a cold outer layer. The consistency of the assumption of a spatially constant absorption coefficient on which the calculations were based is checked. The Eddington approximation is shown to be in good agreement with the exact solution of the problem. The numerical results are compared with those obtained from algebraic formulae valid in some limiting cases.