Abstract: The invention provides polychelant compounds which are useful for example in diagnostic imaging procedures and which are degradable in vivo to release excretable fragments. Such compounds conveniently are of the formula (I): R.sup.1 (X.sup.1 R.sup.2 ((X.sup.2).sub.p L).sub.n).sub.m (where X.sup.1 is a linker moiety metabolically cleavable to release R.sup.1 X.sup.3.sub.m and X.sup.4 R.sup.2 ((X.sup.2).sub.p L).sub.n fragments where X.sup.3 and X.sup.4 are the cleavage residues of X.sup.1 ; R.sup.1 X.sup.3.sub.m is a biotolerable polymer, preferably a substantially monodisperse polymer and especially one with a molecular weight below 40,000 D, particularly below 30,000 D and especially below 20,000 D, for example a first to sixth generation dendrimer; X.sup.4 R.sup.2 ((X.sup.2).sub.p L).sub.n is a polychelant fragment having a molecular weight below 40,000 D, preferably below 30,000 D, especially below 20,000 D, each such moiety preferably being the same; p is 0 or 1; X.sup.