  Several lines of evidence suggest that early-type galaxies might have been
very dusty during their initial evolutionary stages, characterized by intense
star formation activity. The radio selection has provided, by far, the richest
samples of high-$z$ galaxies, most likely of early type, which may therefore
yield crucial direct information on the period when the bulk of stars were
born. In order to investigate the role of dust in these phases, we have
analyzed recent observations of a number of high-$z$ radio galaxies in the
framework of a self-consistent scenario for the evolution of early type
galaxies. The available data can be fully accounted for by ``opaque'' models
similar to that already used by Mazzei and De Zotti (1994a) to fit the spectrum
of the hyperluminous galaxy IRAS~F$10214+4724$. However, there is still
considerable latitude for models. Good fits can be obtained with galactic ages
ranging from 0.05 to 2 Gyr. Crucial constraints may be provided by ground-based
sub-mm measurements and by observations with the forthcoming Infrared Space
Observatory (ISO).
