The variety of iron hydroxides that are formed in biological systems is considered in terms of prototype products which have been identified in synthetic media. Polynuclear hydroxy-oxo complexes with highly ordered domains are typically obtained in rather acid solutions of ferric ions whereas from strongly alkaline polyalcohol solutions very small polynuclear entities can be isolated. In synthetic solutions, it is practically impossible to prepare iron hydroxides from mononuclear ferric ions. In mammals, however, transferrin releases mononuclear ferric iron into cells. The storage protein, ferritin, is shown to be very selective with regard to the transient species admitted for iron hydroxide deposition. On the other hand, mitochondria are non-selective and use all iron hydroxide complexes for heme synthesis regardless of their nuclearity. Although remote from direct experimental verification, realistic transient iron species can be postulated using the results of studies in synthetic media. Cellular miniaturization via localized iron hydroxide formation in a protein cage is absolutely essential to the kinetic regulation of the iron levels in cells.