  A generalization of the MIT bag boundary condition for spinor matter is
proposed basing on the requirement that the Dirac hamiltonian operator be
self-adjoint. An influence of a background magnetic field on the vacuum of
charged spinor matter confined between two parallel material plates is studied.
Employing the most general set of boundary conditions at the plates in the case
of the uniform magnetic field directed orthogonally to the plates, we find the
pressure from the vacuum onto the plates. In physically plausible situations,
the Casimir effect is shown to be repulsive, independently of a choice of
boundary conditions and of a distance between the plates.
