Batch and continuous experiments of fluidization have been carried out in columns of diameter 76 and 152 nlm. Gas velocities of 0.1-0.8 m/s were used with mixtures of coarse and fine FCC as the feed materials. The total entrainment nux E-Z and the elutriation rate constants K-i infinity* are found to depend on the gas velocity in the form of E-Z proportional to U-n and K-i infinity* proportional to U-m, respectively. The values of n and m depend on the bed compositions and particle size; they become smaller at higher gas velocities. For FCC, ii has an average value of 4.0 in the velocity range 0.2-0.4 m/s and tends to a value of 2 at higher gas velocities. The effect of column diameter depends on the gas velocity profiles and is reduced for column diameters above 0.1 m. Empirical correlations to predict K-i infinity* and E-Z have been developed from the results.