PURPOSE: To obtain a substrate-biologically active compd. bonded body for immunoassay useful for diagnosis, etc., by bringing a water-insoluble carboxylated substrate into contact with a 2-(N-heterocyclyloxy)uronium salt and reacting the substrate with a biologically active compd.

CONSTITUTION: A water-insoluble substrate having reactive carboxyl groups is brought into contact with a 2-(N-heterocyclyloxy) uranium salt of the formula (R and R1-R3 are each H, 1-6C alkyl or 2-6C alkenyl, R and R1 or R2 and R3 together may represent C, S, N and O required to complete a 5- to 7-membered heterocyclic ring together with N to which they are bonded; R4 is sufficient C and hetero atoms to complete a 5- to 10-membered heterocyclic ring together with N bonded to the oxy group; and X- is an acid anion) and then the substrate having active groups is brought into contact with a biologically active compd. having a reactive amino or sulhydryl group. The substrate and the biologically active compd. are bonded by covalent bonds.