FIELD : medicine.SUBSTANCE : invention refers to roentgenology, and can be used as radiopaque means for X-ray examinations of various organs. Method involves precipitation of Fe3O4 magnetite from a solution containing iron (II) and iron (III) salts, with a concentrated solution of ammonia at pH value of the reaction mixture of not less than 10 in the presence of a stabilizer - citric acid, taken at rate of 0.02–0.5 mol per 1 mol of stoichiometric colloidal Fe3O4, which is treated in autoclave at 100?°C for 20–30 minutes, then at 180?°C for 12–16 hours, after which is separated by decantation on external magnet and washed. Adding to obtained settling aqueous ammonia solution, then, citric acid and a tantalum solution in hydrofluoric acid are added to the obtained suspension at pH of the reaction mixture of 9–11. Processing the obtained mixture in autoclave at 100?°C for 20–30 minutes, then at 180?°C for 12–16 hours to obtain precipitate of ferrimagnetic particles of Fe3O4 70–400 nm in size coated with X-ray contrast coating of tantalum Ta2O5. Obtained residue is filtered and washed.EFFECT : technical result is higher stability of obtained magnetoactive radio-opaque agent for sedimentation, improved stability of its aqueous suspension due to optimization of dimensions of its particles with simultaneous increase in strength and high adhesion of their radiopaque coating to ferrimagnetic core.1 cl, 1 tbl, 4 ex