[en] The effect of circular dichroism (CD) is predicted for high-order harmonic generation (HHG) by unpolarized atoms subjected to an intense linearly polarized infrared laser field and a weak (perturbative) elliptically polarized extreme ultraviolet (XUV) field. The CD effect is analyzed within the phenomenological consideration of the HHG amplitude in terms of intrinsic vectors of the problem. We show that the CD effect originates from a specific interference between the real and imaginary parts of the HHG amplitude corresponding to the zero- and first-order of perturbation theory in the XUV field. Conditions optimal for observation of the CD effect are discussed and a quantitative estimate for its magnitude is obtained within the time-dependent effective range model. (paper)