Results of the investigations of the fluidization of the bed of solid spheres with density lower than the liquid density are presented. The solid spheres are set up in the downcomer of the airlift reactor with external circulation. The weak linear increase of the liquid circulation velocity is observed for low gas velocity values. Then the faster exponential increase of the liquid circulation velocity is observed. The change point of the curve course corresponds to the onset of the fluidization. It is observed that the minimum liquid fluidizing velocity does not depend and the minimum gas fluidizing velocity depends on the initial height of the solid bed. The minimum fluidizing liquid velocity as well as the minimum fluidizing gas velocity diminish if the liquid viscosity decreases. Results of the investigations are described in the form of the dimensionless equation which allows to calculate the minimum fluidizing liquid velocity.