Magnetic bead-based lab-on-a-chip systems offer significant advantages compared to more conventional systems, mainly through the possibility of controlled manipulation of the magnetic carriers on-chip. In particular, microfluidic immunoassays using functionalized magnetic beads raise increasing interest. We present here a new approach for performing immuno-agglutination assays on-chip. Our system is based on a quadrupolar magnetic field set-up. Dynamic actuation of a confined plug of functionalized magnetic beads is used for analyte capture in a microchannel. A simple detection method based on the swelling of the released plug after agglutination is presented. We demonstrate the feasibility of on-chip agglutination tests by means of a streptavidin/biotinylated-bovine serum albumin (bBSA) model assay. A detection limit of about 200 pg/mL (3 pM) is achieved.