The changing distributions of collagens and glycosaminoglycans have been studied at the attachments of the medial collateral ligament during postnatal development. The ligament is of particular interest because it has a fibrocartilaginous attachment to the femoral epiphysis, but a fibrous one to the tibial metaphysis. Ligaments were examined in rats killed at birth and at 2, 4, 6, 8, 10, 20, 30, 45, 60, 90 and 120 days after birth. Cryosections were immunolabelled with monoclonal and polyclonal antibodies against types I and II collagen, chondroitin 4 and 6 sulfate, dermatan and keratan sulfate. Although the ligament is attached at both ends to bones that develop from cartilage, there was a striking difference in collagen labelling. Type II collagen was only found in spicules of calcified cartilage in bone beneath the tibial enthesis after ossification had commenced, but there was a continuous band of labelling at all stages of development at the femoral enthesis. Initially, the cartilage at the femoral attachment lacked type I collagen, but by 45 days labelling was continuous from ligament to bone. Continuity of labelling was seen much earlier at the tibial enthesis, as soon as bone had formed. There were also marked changes in glycosaminoglycan distribution. Keratan sulfate was present at both entheses up to 45 days, but only at the femoral enthesis thereafter. Both attachments labelled throughout life for dermatan sulfate, but chondroitin 4 and 6 sulfate were only found at the femoral end. The results suggest that enthesial cartilage at the femoral attachment was initially derived from the cartilaginous bone rudiment but was quickly eroded on its deep surface by endochondral ossification as bone formed at the attachment site. It was replaced by fibrocartilage developing in the ligament. This mechanism allows enthesis cartilage/fibrocartilage to contribute to the growth of a bone at a secondary centre of ossification in addition to dissipating stress at the ligament-bone junction.