  We improve on our earlier dynamical estimate of the virial masses of the
haloes of Lyman-break galaxies (LBGs) at redshift z=3 by accounting for the
effects of seeing, slit width, and observational uncertainties. From an
analysis of the small number of available rotation curves for LBGs we determine
a relation Vc7=(1.9+/-0.2)sigma between circular velocity at a radius of 7kpc,
and central line velocity width. We use this relation to transform the measured
velocity widths of 32 LBGs to the distribution of circular velocities, for the
population of LBGs brighter than R=25.5. We compare this distribution against
the predicted distribution for the 'massive-halo' model in which LBGs pinpoint
all of the highest mass dark matter haloes at that epoch. The observed LBG
circular velocities are smaller than the predicted circular velocities by a
factor >1.4+/-0.15. This is a lower limit as we have ignored any increase of
circular velocity caused by baryonic dissipation. The massive-halo model
predicts a median halo virial mass of 10^12.3 Msol, and a small spread of
circular velocities. Our median estimated dynamical mass is <10^(11.6+/-0.3)
Msol, which is significantly smaller; furthermore, the spread of our circular
velocities is much larger than the massive-halo prediction. These results are
consistent with a picture which leaves some of the most-massive haloes
available for occupation by other populations which do not meet the LBG
selection criteria. The median halo mass recently estimated by Adelberger et
al. from the measured clustering of LBGs is 10^(11.86+/-0.3) Msol. Our
dynamical analysis appears to favour lower masses and to be more in line with
the median mass predicted by the collisional starburst model of Somerville et
al., of 10^11.3 Msol. [abridged]
