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A one-stage preparation of stable aqueous magnetic fluids is reported, whereby colloidal Fe3O4 particles are dispersed using naturally occurring polymers and their derivatives (e.g. gelatine, polygalacturonic acid, carboxymethyl-cellulose and succinylated gelatine) as surfactant materials. Low-toxicity materials have been used to permit possible medical use of the fluids. Using a variety of surfactant concentrations at the time of particle formation, control of particle size has been achieved, and particles as small as 3.0 nm in diameter obtained. Stable fluids with up to 6% Fe3O4 content can be produced, having magnetizations of approximately 2 JT-1 Kg-1 (20 Gauss). Fluids diluted 30-fold remain stable colloids.