  Determing the nature of the bright sub-mm sources and their role in the
process of galaxy formation is likely to be a major focus of extra-galactic
research over the next few years. In particular, we would like to know if these
sources are the progenitors of today's massive elliptical galaxies, or
alternatively arise from short-lived, albeit spectacular starbursts within
haloes of unexceptional mass. This question can be addressed from a number of
different directions, one of which is to compare the masses of sub-mm host
galaxies with those of other known massive high-redshift objects. Here I make a
first attempt to do this by exploring whether the few well-studied sub-mm/mm
sources are consistent with the well-established K-z relation for powerful
radio galaxies. Out to z = 3 this appears to be the case, providing evidence
that bright sub-mm sources are indeed destined to be massive ellipticals. At
higher redshifts there is a suggestion that sub-mm galaxies are signficantly
fainter at K than their radio-selected counterparts, but at present it is
unclear whether this indicates a significant difference in stellar mass or the
increasing impact of dust obscuration on the rest-frame light from the sub-mm
hosts.
