Fetal wound healing is characterized by minimal inflammation, mild fibroplasia and rapid, but organized collagen deposition such that scarring is not apparent. The matrices of fetal wounds differ greatly from adult wounds in that fetal wounds are persistently enriched with hyaluronic acid (HA). It has been shown that a reduction in fetal rabbit wound HA results in an adult-like healing response with increased fibroplasia and neovascularization. These observations suggest that HA can modulate cellular activity in fetal repair. Therefore, this study was designed to define the effect of HA on fetal fibroblast function. Fibroblasts from the skin of fetal rabbits were isolated and maintained in culture medium containing either no HA (controls), 1 microgram/ml, 10 micrograms/ml or 100 micrograms/ml of HA (n = 6 for each group). Fibroblast proliferation was quantitated by DNA content in each culture, and collagen and noncollagen protein synthesis were analyzed by incorporation of [3H] proline into collagenase-digestible and collagenase-nondigestible protein, respectively. At all concentrations tested, HA significantly inhibited fetal fibroblast proliferation (p < 0.02), but stimulated collagen (p < 0.002) and noncollagen protein (p < 0.005) synthesis. These findings provide further evidence that HA affects the function of fetal fibroblasts. Moreover, this study in conjunction with previous in utero findings suggests that HA may have a regulatory influence in scarless fetal healing by affecting cellular function during the repair process.