The mean level of factor VIII procoagulant acitivity (VIII:C) and factor VIII related antigen (VIIIR:AG) was normal in 100 newborn cord plasmas, whereas that of von Willebrand factor (VIIIR:WF) activity was slightly lower than normal. On crossed immunoelectrophoresis, 20 of 50 newborn infants had an increased anodal mobility of VIIIR:AG. When the cord plasma showing an abnormal electrophoretic pattern was mixed with normal plasma, two precipitation peaks with a broad base were found. Similar mixing experiments with the abnormal cord plasma and plasma from a patient with atypical von Willebrand's disease did not normalize the electrophoretic mobility of VIIIR:AG. Gel filtration of the cord plasma with an abnormal electrophoretic pattern of VIII:AG, showed that the three activities were all detected at the position corresponding to a molecular weight of about 800 000. The results suggest the presence of qualitative abnormalities of the factor VIII molecule in half of full-term newborn cord plasma.