- We calculate magneto-optical rotation of nonmonochromatic fields in an optically thick cold atomic medium. We show that the nonmonochromatic nature leads to a nonlinear dependence of the rotation angle on optical density. Using our calculations, we provide a quantitative analysis of the recent experimental results of Labeyrie et al. [Phys. Rev. A 64, 033402 (2001)] using cold [Formula Presented] atoms. 2003 The American Physical Society.