Abstract: Compositions comprising stable, water-insoluble coatings on substrates to which biologically active proteins can be covalently coupled so that the resultant product has the biological properties of the protein and the mechanical properties of the substrate, for example, magnetic properties of a metal support. The resultant product can be utilized in diagnostic immunoassays as an example, and when the metal substrate is magnetic, the product can be removed from liquid media by a magnetic field ensuring that any washing or incubation process can be finished promptly and efficiently.