A liquid-fluidized bed or packed bed with gel beads is attractive as an immobilized-cell bioreactor. The performance of such bioreactors is influenced by the physical behavior of these beads. Three different but related aspects involving the drag force between particles and liquid were studied for five types of gel beads, differing in diameter and density: (1) the terminal settling velocity of a single gel bead, (2) the pressure drop over a packed bed and (3) the voidage in a liquid-fluidized bed. Qualitatively, the same trends in these aspects were observed for gel beads as for conventional solids. Quantitatively, however, these aspects were incorrectly predicted by established models (with one exception). It was found that the drag force between gel bead and flowing liquid is smaller than that for conventional solids. As an explanation, two hypotheses are suggested. The first one attributes the drag reduction to small amounts of dissolved polymer. The second one attributes the smaller drag force to the surface nature of gel beads: gel beads contain over 95% of water and thus can be regarded as 'rigid' water droplets. Hence, the gel bead surface might show water-like properties. As an alternative to drag-coefficient relations for conventional solids, the drag coefficient of a single gel bead in a packed or fluidized bed could successfully be described by adapting an existing relation. The success of this description facilitates a more rational design of packed and fluidized beds of gel beads. (c) 2005 Elsevier B.V. All rights reserved.