The influence of the ligands ethylenediaminetetramethylene phosphonic acid (EDTMP) and citrate (CIT) on the biodistribution of radio-yttrium in rats bearing a DS-carcinosarcoma was compared. 88Y-EDTMP and 87Y-CIT were i.v. injected into the same animals. Faster blood clearance and higher renal excretion were observed for the EDTMP-ligand. Of high practical interest is the reduced liver uptake of radio-yttrium (by one order of magnitude) with the EDTMP complex. Since bone and tumour accumulation is only weakly influenced, high tumour-to-liver ratios (up to 14) were observed. We propose to use EDTMP or similar complex ligands for liver blocking when radionuclides like 90Y, 169Yb, 225Ac or other group 3 elements are to be applied in endoradionuclide therapy technique.