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 Z.sup.1 and Z.sup.2 independently represents a group derived from a biological molecule, each of which is linked to A and B via a nucleophilic moiety; or Z.sup.1 and Z.sup.2 together represent a single group derived from a biological molecule which is linked to A and B via two nucleophilic moieties; A is a C.sub.1-5 alkylene or alkenylene chain; and B is a bond or a C.sub.1-4 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. ##STR00001##