Fibrinogen-bound sialic acid was determined in a large series of normal adult, full-term pre-term cord blood samples. Sialic acid was significantly higher in the full-term cord fibrinogen than controls, and higher in premature than term samples. Enzymatic desialation of full-term fibrinogens corrected the abnormal thrombin times and fibrin polymerisation to normal or near-normal values. There was significant correlation between sialic acid content and thrombin time in full-term cord samples. It is concluded that fetal fibrinogen is characterised by an elevated sialic acid content, and that the degree of hypersialation is a function of gestational age.