Calculations are made of the inversion-asymmetry-induced electric dipole moment of the spin-resonance conduction-band transition in InSb. This moment leads to a second-order nonlinear polarization which resonates for difference-frequency mixing when the incident rediation frequencies differ by the spin-flip frequency. Comparison with the experiments of Nguyen and Bridges confirms that the dominant contribution to far-infrared difference mixing is from the magnetic dipole source. However, it is shown that a value for the InSb band parameter C may, in principle, be determined from the results of mixing experiments. © 1974 The American Physical Society.