The magnetic interaction of two periodic terms in the de Haas – van Alphen effect is examined for both weak and strong interaction [Formula: see text]. It is found that the amplitudes of the combination frequencies (and, in particular, the difference frequency) generated by interaction in the parallel component of M are always very weak, if the amplitude of one of the periodic terms is dominant and the frequency of the subsidiary term is close to an integral multiple of the dominant frequency. For the perpendicular component of M, however, in the limit of strong interaction, the amplitude of the difference frequency of two nearly equal frequencies can be relatively much stronger than for the parallel component, if the anisotropy of the Fermi surface is such that the two frequencies vary appreciably differently with orientation. These results are discussed in terms of their relevance to various experimental situations, in particular to the observation by Joseph and Thorsen (1965) of a strong difference frequency in the perpendicular component of M, arising from the nearly equal central and noncentral belly frequencies in silver.