A high-gradient magnetic separation system has been developed for,capture and retrieval of ultra-low plutonium oxide concentrations. The application of advanced matrix materials and improved methodology has demonstrated the effective collection and recovery of submicron paramagnetic actinide particles with particle concentrations as low as 10(-23) M. Incorporation of multiple passes during recovery of magnetically captured particles improves the system mass balance. Activity balances for plutonium were verified with stringent sampling protocols. Collection and recovery values demonstrate that 99% of the submicron plutonium oxide particles can be accounted for when recycle loops are incorporated into capture and recovery circuits, magnetically captured particles are released by sonication, carrier fluids are organically based, and longer matrix lengths are utilized.