The biomechanical properties of this disc depend upon the composition and organization of the extracellular matrix, in which the most important elements are collagen, proteoglycan composition, and the density and orientation of the collagen fibres. The disc is composed of two thickened bands connected by a thin intermediate zone and fibrous attachment regions. Immunohistochemical analysis with monoclonal antibodies to chondroitin-6-sulphate and keratan sulphate revealed a concentration of these cartilage-characteristic glycosaminoglycans surrounding rounded, cartilage-like cells in the bands. Cells were isolated from the cartilage-like band areas and from the fibrous-attachment regions and cultured in vitro. Labelled proteoglycans synthesized by the band cells were similar to those known to be synthesized by hyaline cartilage, but the attachment-region cells synthesized fibroblast-like proteoglycans.