  The half-filled two-dimensional Hubbard model in presence of a uniform and
static parallel magnetic field has been studied by means of the Composite
Operator Method. A fully self-consistent solution, fulfilling all the
constrains coming from the Pauli principle, has been found. The relevant
features of a metal-insulator transition in presence of a magnetic field have
been analyzed. The results qualitatively agree with the ones recently obtained
by means of experimental investigations.
