Link protein, a glycoprotein, that is present both in cartilaginous and non-cartilaginous tissues, has previously been shown to bind to collagen and to proteoglycan. Here, we have examined the effects of link protein and proteoglycans, both alone and in combination, on the assembly of type I collagen fibrils in vitro. Link protein alone had no effect on the kinetics of fibril formation or on the size of the fibrils. Link protein, however, modulated the effects of various proteoglycans including those from bone, cartilage, cornea and sclera. Link protein had the most significant effect on fibril assembly in the presence of the low molecular weight bone proteoglycan. Although the bone proteoglycan alone had no effect on fibril formation, the fibrils were wider in the presence of link protein and proteoglycan. Cartilage proteoglycan alone increased the extent of fibril formation and the resultant fibrils were wider in diameter with a complement of incompletely assembled fibrils. In the presence of both link protein and cartilage proteoglycan, the fibrils were fully formed with the characteristic banding pattern. Further, corneal and scleral proteoglycans alone decreased the extent of fibril formation and the width of the fibrils was either unaltered or slightly decreased in the presence of the link protein. Our results indicate that both link protein and tissue-specific proteoglycans may regulate the organization of collagen fibrils in tissues.