The effect of lanthanum ion substitution for iron on the structural and magnetic properties of Ni-Cu-Zn ferrite is reported. The (Ni0.25Cu0.20Zn0.55)LaxFe2-xO4 ferrite compositions with x = 0.0, 0.025, 0.050 and 0.075, were synthesized by nitrate-citrate autocombustion route. Rietveld structure refinement was carried out to evaluate; La solubility in spinel, residual stress in sintered core, quantity of secondary LaFeO3 phase formed and change in lattice parameters, etc. Density, crystallite size, grain size, residual macrostress and initial permeability were directly affected by the substitution. A significant increase in initial permeability was achieved by a small fraction of La substitution. The La solubility in the Ni-Cu-Zn ferrite lattice was found very low (similar to 0.1 atom/unit cell). Co-relations between magnetic properties and measured physical/structural properties were discussed. (C) 2006 Elsevier Ltd. All rights reserved.