Abstract: An oxidized complex comprising a water-soluble carboxypolysaccharide and magnetic iron oxide, which is characterized by being obtained by oxidizing a complex comprising a water-soluble carboxypolysaccharide and magnetic iron oxide. Said oxidized complex has very low toxicity, is very stable, exhibits little or no blood pressure-reducing action and has low platelets-aggregating action when administered directly into the blood vessels of animals, and is useful in medical fields such as contrast medium for magnetic resonance imaging, exothermic medium for hyperthermia and the like.

Abstract: A composition containing magnetic metal oxide ultrafine particles, which comprises an aqueous sol of a complex of the magnetic metal oxide ultrafine particles with a polysaccharide, a polysaccharide derivative and/or a protein; and an organic monocarboxylic acid. The composition is free of side effects such as platelet aggregation, is safe for use with living bodies, and, in the case of intravascular administration, has no harmful effects on living bodies, and is useful in fields such as medicine and diagnostic drugs.