OBJECTIVE Recent studies from our laboratory have identified the nonaggregating, collagen-binding proteoglycans, fibromodulin (FM) and decorin, and fibronectin (Fn) and albumin, noncovalently bound at the articular surface of cartilage. The present studies were designed to investigate the interactions between these cartilage macromolecules and the underlying collagen matrix and their role as a barrier to cell adhesion in intact articular cartilage. METHODS Cell adhesion studies were carried out with human skin fibroblasts incubated on the articular surface of bovine cartilage explants and on collagen-coated and/or Fn-coated plastic surfaces. Interactions of collagen and Fn with either FM or decorin were studied by radioimmunoassay of the same surfaces, using specific antibodies. RESULTS The present studies show that 1) Fn is immunologically detectable at the intact articular surface of cartilage; 2) fibroblast adhesion to Fn is inhibited by cartilage surface extract proteins and by purified FM, but not by purified decorin; 3) FM has binding affinity for Fn; 4) FM interferes with the binding of a monoclonal antibody specific for the cell-binding domain of Fn; and 5) FM and decorin inhibit collagen-dependent fibroblast adhesion. CONCLUSIONS These results indicate that the small proteoglycans at the normal articular surface may act as a barrier to cell adhesion. Since protective cartilage surface proteins break down readily after the induction of acute arthritis in experimental animals, and in rheumatoid cartilage specimens, it is postulated that proteolytic degradation of the surface proteoglycans may be responsible for increasing cell adhesion to, and subsequent pannus invasion of, articular cartilage in inflammatory arthritis.