Based on the two-band model, we present a transfer-matrix treatment of the tunnel conductance and magnetoresistance for tunneling through ferromagnet/insulator (semiconductor) single junctions and double junctions subject to a dc bias. Our results are qualitatively in agreement with the experimental measurements for the single junction. For the double junction, we find that there exists, spin-polarized resonant tunneling and giant tunnel magnetoresistance. The highest value of the magnetoresistance in a double junction can reach 90%. We anticipate that our results will stimulate some interest in experimental efforts in designing spinpolarized resonant-tunneling devices.