By employing the exact time-dependent wavefunctions for an electron gas under a quantizing magnetic field and laser radiation, we study the dielectric properties of a system when the laser frequency equals the cyclotron frequency. This resonant condition leads to a new magneto-photon-plasmon (magnetopolariton) mode. Unlike the ordinary magnetoplasmon, the energy of the new mode increases linearly with the radiation intensity. As the wavenumber increases, this new mode behaves like a sound wave. The absorption due to the plasmon mode,Jfrm[1/epsi(q,w)], is presented.