Abstract: The object of the invention is a method for preparing a compound of formula I ##STR1## wherein Me.sup.m+ is a lanthanide and n is a number from 1-3, is used for diagnosis of neoplasms. The compound is prepared by adding an aqueous solution of hydrazide hydrochloride to potassium hexachloroplatinate, the suspension is mixed about two hours at 50.degree. to 60.degree. C. then about 30 minutes at 80.degree. to 90.degree. C., and upon cooling the reaction mixture, the unreacted potassium hexachloroplatinate is separated, the solution is decanted and filtered, and the pH of the filtrate is brought to 3 to 5. Methionine is then added in a molar ratio of methionine to tetrachloroplatinate of 2:1-4:1 and upon careful mixing for about 30 minutes a metal salt is added and preferably ytterbium chloride, and tagged with Yb.sup.-169. The components are then mixed and the pH of the solution is brought to 4-6.5 and filtered through a sterile filter.