T lymphocytes up-regulate the synthesis of ligands for E-and P-selectin during proliferative responses in vivo and in vitro. Previous studies from our laboratories indicated that the α (1, 3)-fucosyltransferase FucT-VII regulates the synthesis of E-selectin ligands and sialylated Lewis x-related epitopes (sLe x-related epitopes) in human T lymphoblasts. The current report shows that production of both P-and E-selectin ligands is FucT-VII dependent, but peak synthesis of each occurs at different levels of fucosyltransferase activity in intact cells. In brief, FucT-VII mRNA levels were higher in cultured T lymphoblasts expressing sLe x-related epitopes and both selectin ligands than in cells expressing P-selectin ligands alone. However, synthesis of the epitopes and both selectin ligands required the FucT-VII enzyme in transfected Molt-4 cells. In contrast, neither constitutive nor transfection-enhanced levels of the FucT-IV enzyme generated active P-selectin ligands in these lines. In addition, targeted deletion of the FucT-VII gene in mice markedly inhibited the synthesis of both P-and E-selectin ligands during blast transformation in vitro. Finally, the optimal synthesis of active P-selectin ligands occurred at lower level of FucT-VII activity than required for synthesis of equally active E-selectin ligands in both cultured T lymphoblasts and FucT-VII transfectants. Consequently, the FucT-VII enzyme is essential for the synthesis of both P-and E-selectin ligands by T lymphoblasts, and its activity determines whether P-selectin ligands are expressed alone or in conjunction with E-selectin ligands and sLe x-related epitopes on human T cells.