A composition containing ultrafine particles of a magnetic metal oxide, characterized by comprising an aqueous sol of a composite consisting of the ultrafine particles and a polysaccharide, its derivative and/or a protein, and an organic monocarboxylic acid. This composition is useful in the fields of medicines, diagnostic chemicals and so forth, because it is free from side effects such as platelet agglutination, is excellent in the safety for living organism, and does not adversely affect the living organisms even when administered in the blood vessel.