A permanent human cell line, EA-hy 926, was established by fusing human umbilical vein endothelial cells with the A549 cell line. This hybrid cell line expresses factor VIII-related antigen (VIII:AR), a highly differentiated function of vascular endothelium. The hybrid cells maintain expression of VIII:AR for over 100 population doublings, indicating their permanence. VIII:AR is an endothelial cell product involved in platelet aggregation and is also found in megakaryocytes. It circulates as a large molecular complex and is associated with factor VIII coagulant activity. The hybrid cells exhibit intracellular distribution of VIII:AR similar to primary endothelial cells and accumulate the antigen in culture fluid. Chromosome analysis showed that the hybrid cells have a chromosomal composition combining markers from both parent cell lines. The hybrid cells have vigorous growth properties and can survive beyond the typical lifespan of non-permanent cell lines. The sustained expression of VIII:AR in these hybrid cells suggests that they represent a permanent cell line. This study demonstrates that somatic cell hybridization can be used to immortalize differentiated functions, providing a valuable tool for studying endothelial functions and hemostasis. The results highlight the potential of hybrid cells in maintaining and studying complex biological functions.A permanent human cell line, EA-hy 926, was established by fusing human umbilical vein endothelial cells with the A549 cell line. This hybrid cell line expresses factor VIII-related antigen (VIII:AR), a highly differentiated function of vascular endothelium. The hybrid cells maintain expression of VIII:AR for over 100 population doublings, indicating their permanence. VIII:AR is an endothelial cell product involved in platelet aggregation and is also found in megakaryocytes. It circulates as a large molecular complex and is associated with factor VIII coagulant activity. The hybrid cells exhibit intracellular distribution of VIII:AR similar to primary endothelial cells and accumulate the antigen in culture fluid. Chromosome analysis showed that the hybrid cells have a chromosomal composition combining markers from both parent cell lines. The hybrid cells have vigorous growth properties and can survive beyond the typical lifespan of non-permanent cell lines. The sustained expression of VIII:AR in these hybrid cells suggests that they represent a permanent cell line. This study demonstrates that somatic cell hybridization can be used to immortalize differentiated functions, providing a valuable tool for studying endothelial functions and hemostasis. The results highlight the potential of hybrid cells in maintaining and studying complex biological functions.