The paper describes certain properties of manganese-zinc ferrite cores used in inductor coils. In particular, the stability characteristics which appear to be substantially different from those of iron or alloy dust cores are examined. It is pointed out that the permeability variations are due to several external causes. Only the principal features of these phenomena may be given because of the large quality dispersion of ferrites.Owing to its very reduced losses the manganese-zinc ferrite is the best material for low-induction coils used in the zero to some hundred kilocycles per second frequency range. However, its use requires very accurate knowledge of the different aspects of the stability phenomenon.Some measurement results illustrate the typical behaviour of essential ferrite characteristics.