Improvements in the existing procedures and materials for conduct of high gradient magnetic separation (HGMS) are disclosed. Superior superparamagnetic particles, optionally coated with a polysaccharide or other, usually organic, materials can be prepared in uniform compositions with homogeneous magnetizations. The coating can conveniently be conjugated to a specific binding moiety complementary to a biological material whose purification or separation is desired. In addition, plastic coated matrices which form superior magnetic gradient-intensifying supports are disclosed, along with improved methods and apparatus to conduct HGMS.