A method for production of225Ac is described. Uranium oxide was irradiated with 650 MeV protons. The radium fraction was separated by precipitation with BaSO4, the isotopically pure225Ac as daughter product was then isolated by means of cationexchange chromatography. The biological behaviour of225Ac as a potential candidate for radionuclide tumor therapy is compared with the behaviour of other Me3+-citrate complexes. The actinium shows the faster blood clearance and highest liver uptake in rats and tumor-bearing mice. The tumor uptake is smaller in comparison to ytterbium as a representative for the heavy rare earth elements and slightly higher compared to promethium as representative for the light rare earth elements. In general the biodistribution of actinium is in good agreement with the radiums alteration of Me3+-ions.