Transport through a small metal island attached to two ferromagnetic reservoirs by tunnel junctions is considered. The discrete energy levels due to size quantization, the Coulomb charging energy and the non-equilibrium spin accumulation due to the spin-dependent tunneling rates are taken into account. Analytical results for the zero-bias conductance and magnetoresistance are found. In the nonlinear current–voltage regime, numerical calculations reveal discrete jumps in the tunnel magnetoresistance when the applied voltage is in resonance with the energy difference associated with tunneling of an electron into the ground or excited states of the quantum cluster. Effects of spin-dependent discrete energy levels in magnetic quantum clusters on the tunnel magnetoresistance are studied in detail.