A cell concentration microdevice for immunomagnetic pathogen isolation from a dilute sample is presented. Cells are driven by integrated on-chip pumps through a fluidized bed of immobilized immunomagnetic beads. Off-chip polymerase chain reaction and capillary electrophoretic analysis are used to determine capture efficiencies of E. coli and to optimize the system. Beads are immobilized after each split in a bifurcated channel system to ensure a balanced distribution of beads in all the capture channels. The addition of a pumping flutter step to repeatedly drive sample through the bead bed was found to enhance capture. Capture efficiencies of 70% and a limit of detection of 2 cfu/mu L were achieved; specific capture of E. coli at a concentration of 100 cfu/mu L in a 100-fold background of S. aureus is shown. This capture/concentration system is an important step in overcoming the macro-to-micro interface challenge in the development of microdevices for pathogen detection.