stable suspensions of coated magnetic particles , preferably resuspendable bioactive particles particularly useful in magnetic resonance imaging , are produced by disrupting , what are presumed to be , crystalline agglomerates of a parent particulate magnetic starting material in the presence of a coating material , such that coating can take place during the disruption . the particles are generally coated in suspension yielding a stable suspension of subdivided particles . with the proper selection of a coating material , preferably a protein or other biochemically or biologically active polymer such as an antibody , a resuspendable bioactive product is obtained .