P-selectin glycoprotein ligand 1 (PSGL-1) is a sialomucin expressed on leukocytes that mediates neutrophil rolling on the vascular endothelium. Here, the role of PSGL-1 in mediating lymphocyte migration was studied using mice lacking PSGL-1. In a contact hypersensitivity model, the infiltration of CD4⁺ T lymphocytes into the inflamed skin was reduced in PSGL-1–deficient mice. In vitro–generated T helper (Th)1 cells from PSGL-1–deficient mice did not bind to P-selectin and migrated less efficiently into the inflamed skin than wild-type Th1 cells. To assess the role of PSGL-1 in P- or E-selectin–mediated migration of Th1 cells, the cells were injected into E- or P-selectin–deficient mice. PSGL-1–deficient Th1 cells did not migrate into the inflamed skin of E-selectin–deficient mice, indicating that PSGL-1 on Th1 cells is the sole ligand for P-selectin in vivo. In contrast, PSGL-1–deficient Th1 cells migrated into the inflamed skin of P-selectin–deficient mice, although less efficiently than wild-type Th1 cells. This E-selectin–mediated migration of PSGL-1–deficient or wild-type Th1 cells was not altered by injecting a blocking antibody to L-selectin. These data provide evidence that PSGL-1 on Th1 cells functions as one of the E-selectin ligands in vivo.