  We reassess the hypothesis that Lyman-break galaxies (LBGs) at redshifts z~3
mark the centres of the most massive dark matter haloes at that epoch. First we
reanalyse the kinematic measurements of Pettini et al., and of Erb et al., of
the rest-frame optical emission lines of LBGs. We compare the distribution of
the ratio of the rotation velocity to the central line width, against the
expected distribution for galaxies with random inclination angles, modelled as
singular isothermal spheres. The model fits the data well. On this basis we
argue that the central line width provides a predictor of the circular velocity
at a radius of several kpc. Assembling a larger sample of LBGs with measured
line widths, we compare these results against the theoretical Lambda-CDM
rotation curves of Mo, Mao & White, under the hypothesis that LBGs mark the
centres of the most massive dark matter halos. We find that the circular
velocities are over-predicted by a substantial factor, which we estimate
conservatively as 1.8+/-0.4. This indicates that the model is probably
incorrect. The model of LBGs as relatively low-mass starburst systems, of
Somerville, Primack, and Faber (2001), provides a good fit to the data.
