We extend the well known phenomenon of magnetoresistance (extra resistivity of materials in transverse magnetic field) to a regime where in addition to a transverse magnetic field, a transverse microwave field of resonant frequency is also applied. In a magnetic field, electron spin levels are Zeeman split. In a resonant microwave field, we uncover a new channel of momentum relaxation in which electrons in upper Zeeman level can deexcite to lower Zeeman level by generating spin fluctuation excitation in the lattice (similar to what happens in Electron Spin Resonance (ESR) spectroscopy). An additional resistivity due to this mechanism is predicted in which momentum randomization of Zeeman split electrons happen via bosonic excitations (spin fluctuations). An order of magnitude of this additional resistivity is calculated. The whole work is based upon an extension of Einstein’s derivation of equilibrium Planckian formula to near equilibrium systems.