Four types of mammalian fibres have been studied using various optical techniques with the light microscope. The effects of mechanical pressures upon the fibres have been investigated and their disintegration has been followed with the optical microscope. Parallel studies of the mechanical disintegration resultant from the grinding to produce suspensions for the electron microscope show that at this level these fibres are composite bodies and produce two well defined types of residues. The whole of the work reported has been based upon experiments with individual fibres teased from tissue and isolated manually from surrounding collagen. The absence of the characteristic 640 R striations in the electron microscope shows that these fibres are not of the collagen type and indicates that they are free from contaminants. Recent work has established that two of these fibre types give characteristic cellulose X-ray diffraction patterns, and this, together with morphological and chemical evidence, indicates a polysaccharide structure similar to natural cellulose.