The invention relates to a conjugate of a protein or peptide with a therapeutic, diagnostic or labelling agent, said conjugate containing a protein or peptide bonding portion and a polyethylene glycol portion； in which said protein or peptide bonding portion has the general formula:in which Pr represents said protein or peptide, each Nu represents a nucleophile present in or attached to the protein or peptide, each of A and B independently represents a C1-4alkylene or alkenylene chain, and W′ represents an electron withdrawing group or a group obtained by reduction of an electron withdrawing group； and in which said polyethylene glycol portion is or includes a pendant polyethylene glycol chain which has a terminal end group of formula —CH2CH2OR in which R represents a hydrogen atom, an alkyl group, or an optionally substituted aryl group. Also claimed are a method for making such a conjugate, and novel reagents useful in that method.
