Human endothelial cells synthesize and release antihemophilic factor (AHF, Factor VIII) antigen, as demonstrated by immunofluorescence and radioimmunoassay. Cultured endothelial cells contain AHF antigen, which is released into the culture medium. In contrast, cultured smooth muscle cells and fibroblasts do not exhibit this property. Radioactive amino acids are incorporated into AHF antigen-rich protein fractions from the culture medium, indicating that endothelial cells synthesize and release a protein sharing antigens with normal human AHF. However, no AHF procoagulant activity was detected in the culture medium. Studies suggest that exogenous AHF procoagulant activity is not inactivated by the tissue culture system. The results indicate that endothelial cells actively synthesize AHF antigen, but the absence of procoagulant activity may be due to factors such as inactivation by proteases, immature cells, or incomplete synthesis of the functional molecule. The findings confirm that endothelial cells produce a protein with the same antigens as plasma AHF, supporting the role of endothelial cells in AHF synthesis and release. These results extend previous immunofluorescence studies and suggest that the anatomical distribution of AHF antigen in vascular cells may be important for its function in hemostasis. Further studies are needed to fully understand the mechanisms of AHF synthesis and release by endothelial cells.Human endothelial cells synthesize and release antihemophilic factor (AHF, Factor VIII) antigen, as demonstrated by immunofluorescence and radioimmunoassay. Cultured endothelial cells contain AHF antigen, which is released into the culture medium. In contrast, cultured smooth muscle cells and fibroblasts do not exhibit this property. Radioactive amino acids are incorporated into AHF antigen-rich protein fractions from the culture medium, indicating that endothelial cells synthesize and release a protein sharing antigens with normal human AHF. However, no AHF procoagulant activity was detected in the culture medium. Studies suggest that exogenous AHF procoagulant activity is not inactivated by the tissue culture system. The results indicate that endothelial cells actively synthesize AHF antigen, but the absence of procoagulant activity may be due to factors such as inactivation by proteases, immature cells, or incomplete synthesis of the functional molecule. The findings confirm that endothelial cells produce a protein with the same antigens as plasma AHF, supporting the role of endothelial cells in AHF synthesis and release. These results extend previous immunofluorescence studies and suggest that the anatomical distribution of AHF antigen in vascular cells may be important for its function in hemostasis. Further studies are needed to fully understand the mechanisms of AHF synthesis and release by endothelial cells.