  We calculate magneto-optical rptation of spectrally impure fileds in an
optically thick cold atmic medium. We show that the spectral impurity leads to
non-linear dependence of the rotation angle on optical density. Using our
calculations, we provide a quanttative analysis of the recent experimental
results of G. Labeyrie et al. [Phys. Rev. A 64, 033402 (2001)] using cold
Rb$^{85}$ atoms.
