Osteoarthritis is a degenerative joint disease, characterized by the destruction of the articular cartilage. One of the first changes in the osteoarthritic articular cartilage is a reduction in proteoglycan content. In this study we demonstrate that transforming growth factor beta (TGF beta), a multi-functional growth factor, stimulates the proteoglycan synthesis of explants from human articular knee cartilage dose-dependently in vitro. Osteoarthritic cartilage proved to be much more sensitive to stimulation by TGF beta than normal healthy cartilage. This may indicate that TGF beta plays an important role in the repair of osteoarthritic cartilage.