Gas bubbles are suppressed in fluidized beds of Geldart B magnetic particles when a homogenous magnetic field is applied due to the formation of straight-chain aggregates. Expansion of the non-bubbling bed depends on the orientation of the applied field with respect to gas flow. Hindered bed expansion is reported for axial fields, whereas enhanced expansion is observed for transversal fields. In the present paper we propose a modified Richardson-Zaki equation that takes into account aggregation and chain orientation to describe bed expansion as the gas flow is increased. The only free parameter in this equation is the chain length, that is directly related to the field intensity through a magnetic granular Bond number, defined as the ratio of magnetic to gravitational energy per unit volume. Results from the proposed model are successfully compared with published data in the literature. (c) 2007 Elsevier B.V. All rights reserved.