SUMMARY Although the liver is playing a major role in factor VIII synthesis, extrahepatic sources of factor VIII have been postulated because transplanted hemophilic livers failed to make normal dogs hemophilic. By grafting normal lungs into severely affected hemophilic dogs it was demonstrated that small but definite amounts of factor VIII activity are being produced by the lung. These findings are compatible with the in vitro observations that factor VIII-like antigen is being synthetized by endothelial cells in culture.