The net-field polarization of a plane wave propagating in a magnetically biased cold plasma has been treated in a systematic fashion. Under quasi-longitudinal conditions the polarization transformations are particularly simple and as such have been treated in the literature. However, under more general conditions the polarization transformations are relatively complex. The axial ratio, tilt angle, rotation rate, and polarization sense all become functions of the initial conditions and the constants of the medium. Under quasi-longitudinal conditions the polarization ellipse rocks between two extremes, while the axial ratio also changes periodically between a minimum and a maximum value (Cotton-Mouton effect). The wide variety of polarization transformations available suggests the usefulness of slabs of magnetically biased plasma to synthesize microwave polarizers and depolarizers. The theory of net-field polarization when applied to data obtained during the 1968 solar occultation of Pioneer 6 verified that the usual quasi-longitudinal approximation (Faraday rotation) is adequate to describe S-band polarization in the solar corona.