Human diploid fibroblast (WI-38) in monolayer culture were treated with exogenous glycosaminoglycans for short (up to 4 days) or long (several weeks and months) periods, and the effects on growth and glycosaminoglycan synthesis, as measured by the incorporation of 35S-sulfate and 14C-glycosamine into cell-bound and cell-released (medium) glycosaminoglycans, were determined. Short- and long-term exposure to chondroitin-4-sulfate, chondroitin-6-sulfate, dermatan sulfate or hyaluronic acid at concentrations up to 100 microgram/ml did not affect cell growth, while heparin (between 20 and 100 micrograms/ml), heparan sulfate (above 100 micrograms/ml) or hyaluronic acid (2500 micrograms/ml) exerted significant growth-inhibitory effects. While short-term or long-term influence (each at 100 micrograms/ml) of chondroitin-4-sulfate, chondroitin-6-sulfate and hyaluronic acid resulted in a slight inhibition of incorporation of both radioactive precursors into cell-bound glycosaminoglycans, heparin (between 20 and 500 micrograms/ml) or heparan sulfate (at 100 or 500 micrograms/ml) significantly stimulated 14C-glycosamine incorporation into cell-bound glycosaminoglycans, what appeared to be predominantly into the hyaluronic acid fraction. Following long-term treatment with heparin at 20, 50 or 100 micrograms/ml, incorporation rates of both 14C-glucosamine and 35S-sulfate into both cell-bound and cell-released (medium) glycosaminoglycans were elevated, suggesting a general stimulation of glycosaminoglycan synthesis. Possible mechanisms for the action of these compounds (especially heparin) were discussed, e.g. an interaction with specific cell surface-associated sites.