The 1- to 2-h illumination of xylanase with visible polarized light (PL) prior to the action of that enzyme upon beechwood xylan significantly increased its activity. The activity only negligibly decreased on 3 months storage. The hydrolysis of xylan proceeded in three well-distinguished stages. In the first and fastest stage the effect of illumination was only slightly positive. The effect of the stimulation was noted in the second, slower stage. Enzyme stimulated with PL, preferably by means of the 2-h illumination, performed better than enzyme stimulated with nonpolarized light and non-stimulated enzyme. In the last, the slowest stage, the rates of the reaction were nearly the same using either stimulated or non-stimulated enzyme.