This paper reports experimental observations of an unexpected lift-like interaction force. This force has no apparent analog for single particles in infinite fluids and appears to be a result of multiparticle interactions. The magnitude of this force can be a significant fraction (e.g. 40%) of the total weight of particles in the channel and thus may play a role in flow regime transitions. A simple analysis indicates that the force arises from Bernoulli interaction similar to the mechanism that generates an attraction between particles in a cross-flow. (C) 2004 Elsevier Ltd. All rights reserved.