The hold-up of particles has been studied in two open fluidized beds of 89 and 300 mm diameter. Particles of different materials, density from 1228 to 11400 kg/m(3) and size from 50 mu m to 1 mm, were fluidized. Four distributors were examined in the larger diameter bed - three perforated plates each perforated by holes of 0.8 mm in diameter, but with different hole densities, and a porous plate of the same type as that fixed in the small diameter bed.