Fibronectin, a large-molecular-weight glycoprotein present on cell surfaces and in human plasma, promotes cell adhesion and may modulate reticuloendothelial clearance of particulate debris and bacteria. Amniotic fluid is known to contain a heavily glycosylated variety of fibronectin, and cells derived from amniotic fluid synthesize and secrete fibronectin in tissue culture. The purpose of this study was to determine the relationship between the concentration of fibronectin in amniotic fluid and gestational age. Amniotic fluid samples, obtained from 54 women whose pregnancies ranged in gestation from 15-40 weeks, demonstrated a significant decrease in fibronectin levels with increasing duration of pregnancy (r = -0.70). Diminished concentrations of amniotic fluid fibronectin in the latter stages of pregnancy may represent either decreasing synthesis by amniocytes or a dilutional effect from fetal urine.