Bovine collateral ligament was found to have a water content of 67.5 +/- 2.5%, the tissue was highly collagenous containing 100.3 +/- 15.1 micrograms hydroxyproline/mg dry weight. Type I collagen was the major collagen present with small amounts of Type III and V. The hexuronate content of the tissue was found to be 2.62 +/- 0.40 micrograms hexuronate/mg dry weight of tissue. On incubation in vitro collateral ligament incorporated [35S]sulfate and [3H]acetate into proteoglycans and [3H]acetate into hyaluronate and glycoproteins. The rate of synthesis of proteoglycans by collateral ligament was shown on a weight basis to be greater than that of tendon but lower than that of articular cartilage. Analysis of the proteoglycans present in collateral ligament showed two populations of proteoglycans to be present. Approx. 20% of the total proteoglycans present were large chondroitin- and keratan sulfate-containing proteoglycans capable of forming aggregates with hyaluronate. The major species of proteoglycan present were small dermatan sulfate proteoglycans made up of a core protein with a molecular mass of 45,000 daltons with one dermatan/chondroitin sulfate glycosaminoglycan chain of 30,000 daltons attached. The N-terminal amino acid sequence of the core protein of this proteoglycan showed it to be analogous to the core protein of dermatan sulfate proteoglycan II.