The purpose of this study was to determine if articular cartilage was able to accumulate fibronectin, a large molecule of 440,000 daltons, from the external medium, and if so, to compare the extent of accumulation by normal and osteoarthritic cartilage and to localize the sites of fibronectin accumulation within the articular cartilage. The uptake of canine serum albumin, another protein present in plasma and synovial fluid with a lower molecular weight (67,000 daltons) and a lower pI, was compared. Purified plasma fibronectin and canine albumin were labelled with 125I or N-hydroxysuccinimidobiotin by standard procedures and incubated with articular cartilage explants. The 125I-fibronectin that had bound to cartilage components was extracted with 4 M urea, and both extract and cartilage residues were counted. Cartilage accumulated fibronectin to a greater extent than albumin. For normal cartilage, a level of saturation appeared to be reached at an external concentration for fibronectin of about 150 micrograms/ml. Degenerated cartilage accumulated about 10-fold more fibronectin than normal cartilage. Biotinylated fibronectin was localized within frozen sections of articular cartilage by probing with peroxidase-linked avidin. Fibronectin accumulation in normal cartilage was restricted to the articular surface and the cut edge. In degenerated cartilage, penetration of fibronectin was more extensive but proceeded only from the articular surface. Staining of adjacent sections with peroxidase-linked antifibronectin antibody confirmed previous observations that endogenous fibronectin is present throughout the cartilage matrix. The possibility that synovial fluid fibronectin could be a source of cartilage fibronectin, especially in degenerated cartilage, was discussed.