The authors investigate low-frequency instabilities caused by non-uniformity of the macroscopic parameters of a plasma located in a constant magnetic and a high-frequency (HF) electric field. In section 2 the instabilities of a completely ionized plasma are studied with allowance for Coulomb collisions. It is shown that the effect of "induced" longitudinal friction significantly changes the spectrum of natural oscillations of the drift-dissipative type. In section 3 the authors consider the influence of the HF field on hydrodynamic drift-temperature, drift-cyclotron and centrifugal instabilities. The HF field is found to have both a stabilizing and a destabilizing influence.