  We study the insulator-superfluid transition of spin-1 bosons in an optical
lattice in a uniform magnetic field. Based on a mean-field approximation we
obtained a zero-temperature phase diagram. We found that depending on the
particle number the transition for bosons with antiferromagnetic interaction
may occur into different superfluid phases with spins aligned along or opposite
to the field direction. This is qualitatively different from the field-free
transition for which the mean-field theory predicts a unique (polar) superfluid
state for any particle number.
