The permeability spectra of composite magnetic materials (NiZn ferrite composites and YIG composites) has been experimentally studied on a wide range in magnetic load C. The resonance frequency decreases with the increasing magnetic load of the mixture. This is attributed to the fading of the magnetic pole on the surface of the magnetic inclusions. A theory is developed which is intended to give an expression for the gyroresonance frequency of magnetic composites in their unsaturated magnetic state. It fits the experimental data over its full range of volume fraction C without any corrective factors. The proposed relation, which is an extension of the Kittel's expression for the resonance frequency of a saturated body, points out a noteworthy close link between the concept of reciprocity and the one of demagnetization.