Abstract: An in vivo method for preparing, inserting, isolating, collecting and assaying diagnostic ligates present in body fluids. Magnetically responsive particles with attached ligands specific for a particular ligate are introduced into the body fluid. Bonding between the ligand and ligate results in particle/ligand/ligate complexes. The complexes are removed by the application of a magnetic field and the complexes are permitted to cluster. The clusters are then quantitated to determine the concentration of the ligate in the body fluid. Associated materials and devices useful in the practice of this method are also provided.