  We investigate theoretically the effects of surface states over the
magnetoresistance of Ni-based organic spin-valves. In particular we perform
{\it ab initio} electronic transport calculations for a benzene-thiolate
molecule chemically attached to a Ni [001] surface and contacted either by Te
to another Ni [001] surface, or terminated by a thiol group and probed by a Ni
STM tip. In the case of S- and Te-bonded molecules we find a large asymmetry in
the spin-currents as a function of the bias, although the $I$-$V$ is rather
symmetric. This leads to a smooth although not monotonic dependence of the
magnetoresistance over the bias. In contrast, in the case of a STM-type
geometry we demonstrate that the spin-current and the magnetoresistance can be
drastically changed with bias. This is the result of a resonance between a
spin-polarized surface state of the substrate and the $d$-shell band-edge of
the tip.
