We have investigated the distribution of vWF multimers in blood from the umbilical cord of infants delivered vaginally and by caesarean section, from heel-stick blood collected 1 d post-partum, and from fetuses undergoing evaluation for Rh compatibility. To examine vWF multimers, plasma was separated by electrophoresis on SDS-agarose gels, overlaid with 125I-anti-vWF, and analysed by densitometry of autoradiographs. Neonatal and fetal plasma contained unusually large von Willebrand factor multimers (ULvWFM), not present in normal adult plasma, in shed blood from adults, in maternal plasma at the time of birth, or in plasma from adults deficient in vitamin K-dependent coagulation proteins. We conclude that ULvWFM, similar in size to vWF present in the Weibel Paladie bodies of endothelial cells, the alpha granules of platelets, and the plasma of patients with TTP, is present in the fetal circulation, at birth, and shortly after delivery.