The effect of PDGF-BB on human gingival fibroblasts was monitored in an in vitro system. PDGF was found to be mitogenic for these cells, although it required the presence of low concentrations of fetal calf serum to be active. Proteoglycan and hyaluronate synthesis was analyzed by labeling newly synthesized macromolecules with [35S]-sulfate or [3H]-glucosamine, respectively. Identification of specific glycosaminoglycans was achieved by selective enzymatic or chemical degradations. It was found that cells cultured in the presence of PDGF showed no discernible differences in proteoglycan synthesis relative to the control cultures. There were no alterations in amounts of proteoglycans synthesized, types of sulfated glycosaminoglycans synthesized, or relative hydrodynamic sizes of the proteoglycans. In contrast to the proteoglycans, hyaluronate synthesis was significantly increased in the presence of PDGF. The increase in [3H]-glucosamine incorporation into newly synthesized hyaluronate correlated with an increase in the activity of the enzyme hyaluronate synthetase but could not be accounted for entirely by changes in the specific activity of sugar nucleotide precursors, which did alter slightly under differing culture conditions. It is concluded from these results that PDGF stimulates gingival fibroblasts to proliferate and is associated with a differential effect of proteoglycan and hyaluronate synthesis. These observations may correlate with the observed early events associated with wound healing and repair.