The filaments of a tyrecord rayon were modified by two methods to enhance the degree of swelling of the material : (1) carboxyl and sulfonic acid substituents were introduced into the rayon and (2) the filaments were swollen in sodium hydroxide. The water-swollen filaments were rendered magnetic by in situ synthesis of ferrites and the resulting magnetic filaments were characterized by transmission electron microscopy (TEM) and vibrating sample magnetometry (VSM). Two other non-ionic, highly swollen cellulose gels were used as matrices for in situ synthesis of ferrites : a never-dried, wet-spun model cellulose filament and a never-dried bacterial cellulose membrane. TEM micrographs of thin cross-sections of the magnetic gels showed that the nanometre-sized ferrites were uniformly distributed whereas the treated rayon filaments had ferrites predominantly at the filament surface. All the materials were superparamagnetic as determined by VSM. However, a ferrimagnetic component was detected after several reaction cycles for the bacterial cellulose membrane by Mossbauer spectroscopy.