We consider the correction to conductivity of a 2D electron gas due to electron-electron interaction in the parallel magnetic field at arbitrary relation between temperature and the elastic mean free time. The correction exhibits non-trivial dependence on both temperature and the field. This dependence is determined by the Fermi liquid constant, which accounts for the spin-exchange interaction. In particular, the sign of the slope of the temperature dependence is not universal and can change with the increase of the field.