Abstract: Biologically active reagents are prepared from particles of copolymers having highly reactive carboxy or equivalent groups. The reagents are prepared by covalently attaching biologically active substances, for example antibodies, to the particles, directly or indirectly through highly reactive carboxy groups on the particle surface. These reagents are used to advantage in analytical elements, methods for the detection of specific binding ligands (such as immunological species) and immunoassays, and in purification methods as affinity chromatography reagents.