An expression for circular dichroism is derived by a method in which the radiation field, in addition to the molecules, is quantized. From the total scattering cross sections for right and for left circularly polarized photons, the respective absorption coefficients are determined. A formula for the circular dichroism is obtained from the difference between the absorption coefficients. The present result is in agreement with the semi-classical result of Condon, Altar, and Eyring.