  The spin-polarized transport through two-level quantum dots weakly coupled to
ferromagnetic leads is considered theoretically in the Coulomb blockade regime.
It is assumed that the dot is doubly occupied, so that the current flows due to
cotunneling through singlet and triplet states of the dot. It is shown that
transport characteristics strongly depend on the ground state of quantum dot.
If the ground state is a singlet, differential conductance ($G$) displays a
broad minimum at low bias voltage, while tunnel magnetoresistance (TMR) is
given by the Julliere value. If triplet is the ground state of the system,
there is a maximum in differential conductance at zero bias when the leads are
magnetized in antiparallel. The maximum is accompanied by a minimum in TMR. The
different behavior of $G$ and TMR may thus help to determine the ground state
of the dot and the energy difference between the singlet and triplet states.
