[en] In low-density universe models the gravitational growth of density perturbations are slower than that in the Einstein-de Sitter critical model, and larger amplitudes are required at the decoupling time, in order that protoclusters may form to the present epoch. In this paper it is shown that, if the thermal model without secondary heating is assumed, the required amplitudes of entropy perturbations exceed the observational limit derived from the high degree of isotropy of the cosmic background radiation, as well as those of adiabatic perturbations. (author)