The present invention is directed to a method for the preparation of a radiolabled conjugate composition comprising Bismuth-213 loaded targeting moieties that have been diluted with cold targeting moieties in a predetermined ratio of amount of cold targeting moieties to amount of Bismuth-213 loaded targeting moieties. Said ratio is determined by determining the amount T of target moieties per diseased cell to be treated, and determining the amount Z of Bismuth-213 atoms needed to kill a diseased cell, and contacting targeting moieties with Bismuth-213 atoms in a ratio of T over Z, to form a conjugate composition with a predetermined ratio of cold targeting moieties and Bismuth-213 loaded targeting moieties.The invention is further directed to a conjugate that can be prepared with the method according to the invention, a pharmaceutical formulation comprising the conjugate according to the invention.
