In large eddy simulations (LES) of radiative heat transfer, the subgrid-scale (SGS) radiative emission and the SGS radiative absorption are two unclosed terms that arise after applying the filtering operation to the radiative transfer equation and which represent the effect of the SGS motions on the evolution of the resolved radiative heat flux. In the present work, a-priori tests based on direct numerical simulation of homogeneous isotropic turbulence have been carried out. It was found that the effect of the SGS radi-ative absorption may be neglected in LES, whereas the SGS radiative emission has to be modelled, particularly for engineering applications where the grids are generally coarse, and in flows with high turbulence intensities. Future works should be devoted to the development of SGS models for radiative transfer.