Abstract: An organo-iron compound or complex, having an X-ray diffraction pattern similar to that of magnetite and containing from 25 to 50 per cent or more iron, is water soluble and exhibits strongly ferromagnetic properties both as a solid and in aqueous solution. The compound is prepared by (1) precipitating a magnetite from an alkaline solution, optionally in the presence of a protein or protein degradation product, (2) optionally treating the precipitate with an oxidizing agent such as hydrogen peroxide, and (3) reacting the precipitate with a hydroxy-carboxylic acid. The resulting material is solubilized by treatment with a base.