Experiments have been performed in which a fluidized bed in a metal container was tilted and observed with X-rays. Previous experiments using such a system emphasized the importance of interparticle forces in determining bed behaviour. In the present work these forces were controlled through the use of a metal container and by varying the humidity of the fluidizing gas. It was found that, as previously reported, when the container was tilted the surface of the bed also tilted; however, unlike in the previous experiments, the angle of tilt of the bed surface was nearly always less than that of the container. The two angles were closer if there was moisture in the fluidizing air. The angle of tilt at which the bed failed decreased with increasing gas velocity. A theory to explain the behaviour of the tilting bed is proposed.