Link proteins have been purified from avian xyphoid process. Cartilage was extracted in 4.0 M guanidine hydrochloride and a link fraction (A1D5) was obtained by sequential cesium chloride centrifugation. Link proteins were separated from low buoyant density proteoglycans by chromatography on Sephacryl S-200 and polyacrylamide gel electrophoresis in sodium dodecyl sulfate. The presence of contaminating proteoglycans at various purification steps was monitored in an enzyme-linked immunosorbent assay using antiserum against avian cartilage proteoglycan monomer (anti A1D1-1400 Vo). Antiserum generated against this purified link preparation (anti link[SDS]) was characterized for its ability to bind link proteins, proteoglycan monomer, and aggregate. The serum was specific only for link proteins when tested by an enzyme-linked immunosorbent assay. Some reactivity against proteoglycan monomer was observed in a Farr-type assay.