The depression of magnetite in cationic flotation has been investigated using external 3D uniform low magnetic flux densities up to 0.015 T, in a laboratory Jameson-type flotation cell and in the absence of chemical depressants. In addition, the magnetic susceptibility and agglomeration of pure magnetite particles have been evaluated. The motion, size and morphology of magnetite aggregates and bubbles-magnetite aggregates in the flotation cell have been monitored by video microscopy. The cationic collector dodecyl amine and an iron concentrate with 65.1% total Fe, mainly as magnetite, were used for the flotation studies. In the absence of a magnetic field, the magnetite particles were fully dispersed and the iron flotation recovery linearly increased with the amine concentration. The 3D uniform magnetic flux density led to aggregation of magnetite particles as chains and chain columns whose sizes increased as the magnetic flux density increased. These magnetite aggregates rapidly settled by the action of the gravitational force and were removed easily from the flotation cell as tailings. The high amine flotation recovery of iron was significantly impaired by the magnetic field. Magnetite depression was associated with the large mass of the magnetite aggregates, which could not be floated by attached air bubbles so that they passed out as tailings.