In situ synthesis of iron oxide particles in O-(carboxymethyl)ated cellulose fibers and sulfonated wood fibers was performed by careful oxidation of ferrous hydroxide precipitated by alkali from the ferrous ion-exchanged form of the matrix. This "biomimetic" chemistry yielded magnetic fibers containing small ferrite (Fe304) particles of -10 nm in size . Magnetic hysteresis-loop measurements showed that the material, in the form of a thin parchment-like film, is superparamagnetic, that is, it possesses no remanent magnetization . The process described herein could be practiced with a wide range of natural biopolymers in order to obtain magnetically responsive materials suitable for such biotechnological applications as magnetic separations . * Diamagnetism 10 is a weak magnetism in which a magnetization is exhibited opposite to the direction of the applied field . In paramagnetism, the magnetization is proportional to the magnetic field . Superparamagnetism" ,12 is a weak magnetism similar to paramagnetism in the sense of exhibiting a small positive susceptibility but, due to thermal agitation, it presents a random orientation of the spins . These materials display little or no remanence and coercivity phenomena characteristic of ferro(i)magnetic substances .