1. A compound of hyaluronate and protein, called hyaluronate-protein was isolated from pooled human synovial fluids by caesium chloride density-gradient ultracentrifugation. 2. The isolated hyaluronate-protein was labelled with [(125)I]iodide and the following studies were done. (a) Ultracentrifugation in caesium chloride showed that the protein moiety ((125)I counts) and hyaluronate (hexuronate) sedimented together in the middle of the gradient. (b) The labelled hyaluronate-protein was treated with trypsin, and ultracentrifugation showed that peptide fragments ((125)I counts) were dispersed throughout the gradient, indicating proteolytic digestion. Hyaluronate sedimented in the middle of the gradient. (c) The labelled hyaluronate-protein was digested with streptococcal hyaluronidase, and ultracentrifugation showed that hyaluronate fragments were dispersed throughout the gradient, indicating digestion of the polysaccharide. The protein moiety, without attached hyaluronate, now sedimented at the top of the gradient. (d) Ultracentrifugation of labelled hyaluronate-protein in 4m-guanidinium chloride showed that protein and hyaluronate sedimented together. 3. These studies confirm that hyaluronate is combined with a small quantity of protein in normal human synovial fluid. A mild method for the rapid isolation of hyaluronate-protein in good yield is described.