The prediction of the minimum fluidization velocity of beds of Geldart's group B particles confined in a packed bed of coarse spheres can be achieved by extending to this peculiar type of systems the theory developed for modelling the behaviour of segregating beds of simultaneously fluidized solids. The approach is based on separate force balances on the fluid and the fine solids, capable to account for the peculiar nature of solid-solid interaction in a confined fluidized bed. Its validation is fulfilled by an extended investigation conducted in two columns (5 and 10 cm OD) packed with a fixed bed of 4.1 mm lead shots or 11 mm glass beads. The effect of particle size on the fluidization regime is investigated by comparing the results provided by experiments in which various cuts of glass ballotini, ranging from 100 to nearly 600 tun, are fluidized in two packings of fixed spheres; possible differences of behaviour due to particle density are analysed by series of experiments employing particles of ceramics, zirconium oxide, steel and bronze of the same diameter. The results obtained confirm the effectiveness of the approach followed, in which an important role is played by the indirect interaction between the two solid phases. (C) 2016 Elsevier B.V. All rights reserved.