Abstract: A compound of formula: ##STR1## in which X.sup.1 is a reactive group that can react to form a covalent bond with a reactive group on the surface of a material to be rendered biocompatible, X.sup.2 is a group -0.sup..crclbar. or a precursor of such a group, n is 2, 3 or 4, Y is a group --N.sup..sym. R.sub.3 A.sup..crclbar. wherein each R, which may be the same or different, is a C.sub.1 -C.sub.4 alkyl group and A.sup..crclbar. is an anion present when X.sup.2 is an electrically neutral group or Y is ##STR2## wherein R.sup.1 together with X.sup.2 forms a direct bond between the nitrogen and the phosphorus atoms.