  The critical current density of four MgB2 samples was measured inductively
and for one of them also by transport. Pure phased and dense bulk samples yield
a critical current density which in value, as well as in its magnetic field and
temperature dependence, is essentially the same as the intra-granular current
density measured in a dispersed powder. Also the correspondence between the
inductive and transport data indicates that the grain boundaries in the bulk
samples are totally transparent for the current. The current-voltage relation
becomes rapidly shallow in the vicinity of a depinning line, well below the
second critical field. Between the depinning line and the second critical field
the material is ohmic and shows a marked magneto-resisitivity, indicative of a
flux-flow regime.
