  We show that the ABC scenario we proposed for the co-evolution of spheroids
and QSOs predicts accretion rates and masses of supermassive black holes in
sub-mm galaxies in keeping with recent X-ray determinations. These masses are
well below the local values, and those predicted by alternative models. The
observed column densities may be mostly due to ISM in the galaxy. The
contribution of the associated nuclear activity to the X-ray background is
likely negligible, while they may contribute a sizeable fraction $\sim 10 %$ to
hard-X cumulative counts at the faintest observed fluxes.
