An arbitrary plane slab is considered within a magneto plasma whose parameters vary in a direction perpendicular to the (horizontal) plane of stratification, the plane of the slab faces. 2 × 2 reflection and transmission coefficient matrices are defined for the slab in terms of up-going waves incident from below, Ru and Tu, and in terms of downgoing waves incident from above, RD and TD. The matrix elements compare amplitudes of the wave fields of the characteristic magnetoionic modes, so normalized that in regions of negligible absorption the squares of the amplitudes are proportional to the mean Poynting vector. The theorem that is proved states that the matrix Tu is the transpose of the matrix TD and that the matrices Ru and RD are symmetric. Some applications are discussed.