We state a criterion for existence of a stored-energy function from which we will derive the stress in the case of microstructures with finite deformations. The problem was examined by Truesdell in the case of classical continuous media and is connected to the possibility of an extension of the classical Betti theorem to the case of finite deformations. Unlike what happens in the case of classical continuous media, we realise that the existence condition for a stored-energy function in the case of microstructure, involves also the stress, and precisely the part that characterizes the internal forces, but not the part concerning couples and hyperstress.