The effects of hyaluronate on the morphology, motile and social behaviour of embryonic chick heart fibroblasts were examined in the presence or absence of hyaluronate binding proteins (HABP). HABP were inserted onto fibroblasts after treating cells with 0.2 M-urea. Such treatment increased the binding of [125I]HABP to cell monolayers: at saturation, binding was increased threefold compared to untreated monolayers. Generally, the distribution of added HABP was similar to that of endogenous HABP as detected by immunochemistry. [3H]hyaluronate bound to HABP-treated monolayers in amounts that were proportional to the amount of HABP added. HABP alone reduced cell motility, decreased cell underlapping (measured as a nuclear overlap ratio) and promoted cell spreading. Hyaluronate added by itself had no significant effect on cell behaviour. However, in the presence of HABP, hyaluronate increased cell motility, reduced cell spreading, promoted the appearance of surface blebbing and ruffles, and significantly increased the nuclear overlap ratio. These biological effects were proportional to the amounts of hyaluronate and HABP that bound to heart fibroblasts. Fibronectin, bovine serum albumin or sulphated glycosaminoglycans added together with HABP did not mimic the effects of hyaluronate and had little effect additional to HABP alone. The role of HABP as a putative cell binding molecule for hyaluronate and the relationship of hyaluronate to cell social and motile behaviour are discussed in the light of these findings.