New and long published experimental observations of the onset of the aggregate (bubbling) behaviour in liquid fluidized beds are shown to be in agreement with the predictions of a recently published model of the fluidisation process. For a given liquid, the transition from particulate to aggregate fluidization depends on both the density and size of the suspended particles: the lower the particle density the larger is the size necessary for a transition to occur: thus for fluidisation by ambient water, lead particles larger than 200 mu m and soda glass particles larger than 2000 mu m will exhibit aggregate behaviour; below a critical particle density no such transition is possible.