The presence, nature, and composition of a layer of transparent mucinous material overlying the posterior epithelium (called here, 'the viscous layer of the posterior epithelium') was investigated in material from domestic cats. A number of different techniques were used. (a) A variety of different fixatives and histochemical stains was used to preserve and visualise the layer of examination by light microscopy. Frozen sections of fresh material were also examined after on-slide fixation. The preservation and appearance of the viscous layer was critically dependent on the type of fixative used and on minimal agitation during tissue processing. It appeared to contain sulphated acid GAGs, the staining of which was histochemically masked around pH 2.5. The use of frozen sections produced the most reliable preservation of the layer. (b) Ruthenium red or cetylpyridinium chloride was used to preserve the viscous layer for examination by transmission electron microscopy. It appeared in a dispersed form after stabilisation by ruthenium red and in a condensed form after stabilisation by cetylpyridinium chloride. (c) Freeze-dried tissue was examined by scanning electron microscopy. The viscous layer usually appeared as a band of coral- or moss-like material which was often many times the thickness of the posterior epithelium. Usually, the areas of the layer adjoining the posterior epithelium and the anterior chamber appeared more condensed when compared to the material between these two regions. The physiological significance of the viscous layer is discussed.