A bead-based sedimentation biodetector is studied theoretically. The biodetector operates with a suspension of settling beads, non-settling reporters, and target analytes-all initially suspended in a buffer solution. The reporters can be either fluorescent molecules or small particles. The functionalized beads interact with the reporters and target analytes while settling under the action of gravitational, electric, and/or magnetic fields. Both sandwich and competitive assays with hindered settling are considered. In the sandwich format, in the presence of target analytes, the reporters bind to the beads and settle (the target analytes provide the link between the beads and the reporters). A reduction in the reporters' concentration indicates the presence of target analytes. In the competitive format, both target analytes and reporters compete for bead-based binding sites. In the absence of target analytes, one would observe a reduction in the suspended reporters' concentration. The model allows one to predict the reporters' concentration in solution as a function of initial bead, reporter, and target analyte concentrations and provides a means for the reactor's optimization. (c) 2005 Elsevier Ltd. All rights reserved.