Novel biologically active compounds of the general formula (I) 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; each of Z1 and Z2 independently represents a group derived from a biological molecule, each of which is linked to A and B via a nucleophilic moiety; or Z1 and Z2 together represent a single group derived from a biological molecule which is linked to A and B via two nucleophilic moieties; A is a C1-5 alkylene or alkenylene chain; and B is a bond or a C1-4 alkylene or alkenylene chain; are formed by conjugating a suitable polymer to a suitable biologically active molecule via nucleophilic groups in said molecule, preferably via a disulphide bridge.