Novel compounds of the general formula in which one of X and X represents a polymer, and the other represents a hydrogen atom each Q independently represents a linking group W represents an electron-withdrawing moiety or a moiety preparable by reduction of an electron-withdrawing moiety or, if X represents a polymer, X-Q-W- together may represent an electron withdrawing group and in addition, if X represents a polymer, X and electron withdrawing group W together with the interjacent atoms may form a ring Z 1 and Z 2 independently represents a protein, each of which is linked to A and B via a nucleophilic moiety, or Z 1 and Z 2 together represent a single protein which is linked to A and B via two nucleophilic moieties, each of said nucleophilic moieties being derived from an amine group A is a C 1-5 alkylene or alkenylene chain and B is a bond or a C 1-4 alkylene or alkenylene chain are formed by conjugating a suitable polymer to a protein via nucleophilic groups in said protein.
