By means of the diagrammatic techniques for nonequilibrium processes proposed by Keldysh and adopting a simple one-band tight-binding Hamiltonian, we study the resonant tunneling in GaAs/AlxGa1–xAs double-barrier heterostructures (DBH) in the presence of a magnetic field applied parallel to the current direction in this work. We have found that the number of Landau levels that contribute to the resonant tunneling diminishes with the magnetic field, all centered around the position of the resonance. Also, we have calculated the conductance versus bias voltage and we have found that the maxima and minima of the conductance move towards higher bias with the magnetic field in good agreement with experimental reports. We have considered the effects of isoelectronic impurity-planes localized at the well region on the J–V characteristics of the system and on the conductance versus voltage. The presence of the impurity-plane induces a shift to higher (lower) energies of the density of states, of the J–V characteristic curves and of the conductance versus voltage, corresponding to a repulsive (attractive) potential.