Preparations of cellular fibronectin from chick embryonic fibroblasts have previously been shown to have hyaluronate-binding activity. However, gel filtration and CsCl isopycnic centrifugation of fibronectin preparations showed that the binding activity was associated with molecules with a density and a molecular weight higher than those of fibronectin. An immunoprecipitation assay using antibodies to the chondroitin sulfate proteoglycan (PG-M) from the mesenchyme of chick embryo limb bud showed that the hyaluronate-binding activity of fibronectin preparations was precipitable with this antibody. The immunoprecipitation analyses also showed that fibronectin preparations as well as conditioned culture medium and extracts of chick embryonic fibroblasts contained a chondroitin sulfate proteoglycan, the protein-enriched core molecules from which were identical to those from PG-M with respect to electrophoretic mobility and immunological reactivity. This proteoglycan was purified from conditioned culture medium and extracts of fibroblasts by dissociative CsCl isopycnic centrifugation. The proteoglycans from medium or extracts gave core derivatives with electrophoretic mobility identical to those from PG-M, and they had equal hyaluronate-binding activities. These results, taken together, suggest that most, if not all, of the hyaluronate-binding activity in preparations of chick cellular fibronectin is due to a proteoglycan identical to PG-M. This proteoglycan was also found to bind directly to fibronectin and to type I collagen, but not to laminin or type IV collagen. It is possible that the fibroblast proteoglycan mediates interactions between hyaluronate, fibronectin, and type I collagen, thereby participating in formation of the pericellular matrix of fibroblasts.