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.
