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SYNOPSIS A resume is given of the literature on the morphology of collagen fibre bundles, and the occurrence in them of elastin, reticular tissue, and a cement substance. Two methods of demonstrating reticular tissue have been investigated. (1) Silver impregnation. A number of variations of the technique have been tried, and Foot's balanced solutions method has been adopted as the most satisfactory for skin. It gives clear, sharp preparations, in which (after toning with gold) collagen becomes rose-red to rather purple in colour, and reticular tissue usually shows black. The fine binding fibrils around collagen fibre bundles did not stain as elastin, nor did they become black when impregnated with silver. Nevertheless, they are considered to be reticular tissue, since morphologically they resemble reticulin more closely than they do collagen. (2) Swelling of fibres. Fibre bundles teased from the skin of cattle (calf and ox) and sheep (3–18 months) were soaked at a range of pH values in hydrochloric acid solutions. Calf fibre bundles from the leg swelled more readily and at higher pH values than those of the ox from the same position, in which the restrictive binding tissue was present as an intermittent sheath. The collagen fibres twisted into a rope-like form and became entangled in the associated elastin fibres, which, therefore, played an important part in restricting the fibre bundles as they swelled. It is also inferred, though the investigation was not specially directed towards this point, that a cement substance is present, holding the collagen fibres into bundles even in the absence of reticular tissue and elastin.