A radioactive diagnostic agent having high labeling efficiency, prepared by bringing an aqueous solution of a radioactive metallic element contaminated with a non-radioactive metallic ions into contact with a non-radioactive carrier comprising a carrier substance having a chelate-forming property and optionally a physiologically active substance chemically bonded thereto, at least one of said aqueous solution and said non-radioactive carrier comprising at least one reductive material chosen from ascorbic acid and erythrobic acid, and their salts and esters.
