Proteoglycans were isolated from bovine skin, sclera, deep flexor tendon and the periphery of the temporomandibular joint disc with urea. Decorin was purified from each of these extracts by ion-exchange, hydrophobic-interaction and gel-filtration chromatography. Purities were assessed by amino acid analysis and by sodium dodecylsulphate polyacrylamide gel electrophoresis (SDS-PAGE) of the protein cores released by digestion with chondroitin-ABC-lyase. In these respects the decorins were indistinguishable. However the glycosaminoglycan chains released by digesting the proteoglycans with papain varied widely in mobility on SDS-PAGE: that from skin decorin migrating fastest and that from tendon decorin slowest. The effects of each of the decorins on collagen fibrillogenesis in vitro were similar, all reducing the rate of fibril growth (by 55 to 71%, depending on the source of the proteoglycan) and increasing the diameters of the fibrils formed (by 27 to 66%). Core protein alone, isolated from skin decorin, reduced the rate of fibril growth as effectively as intact decorin, but had no effect on the diameter of fibrils formed. The dermatan sulphate chain and the protein thus appear to play different roles in the interaction of intact decorin with collagen. These data suggest that decorin found in fibrous connective tissues may increase Type I collagen fibril diameters, resulting in tissues that are better able to withstand tensile forces.