Abstract Purpose: The binding of hematogenously borne malignant cells that express the carbohydrate sialyl Lewis X (sLeX) to selectin adhesion receptors on leukocytes, platelets, and endothelial cells facilitates metastasis. The glycosylation inhibitor, per-O-acetylated GlcNAcβ1,3Galβ-O-naphthalenemethanol (AcGnG-NM), inhibits the biosynthesis of sLeX in tumor cells. To evaluate the efficacy of AcGnG-NM as an antimetastatic agent, we examined its effect on experimental metastasis and on spontaneous hematogenous dissemination of murine Lewis lung carcinoma and B16BL6 melanoma cells. Experimental Design: Tumor cells were treated in vitro with AcGnG-NM, and the degree of selectin ligand inhibition and experimental metastasis was analyzed in wild-type and P-selectin-deficient mice. Conditions were developed for systemic administration of AcGnG-NM, and the presence of tumor cells in the lungs was assessed using bromodeoxyuridine labeling in vivo. The effect of AcGnG-NM on inflammation was examined using an acute peritonitis model. Results: In vitro treatment of Lewis lung carcinoma cells with AcGnG-NM reduced expression of sLeX- and P-selectin-dependent cell adhesion to plates coated with P-selectin. Treatment also reduced formation of lung foci when cells were injected into syngeneic mice. Systemic administration of the disaccharide significantly inhibited spontaneous dissemination of the cells to the lungs from a primary s.c. tumor, whereas an acetylated disaccharide not related to sLeX in structure had no effect. AcGnG-NM did not alter the level of circulating leukocytes or platelets, the expression of P-selectin ligands on neutrophils, or sLeX-dependent inflammation. Conclusion: Taken together, these data show that AcGnG-NM provides a targeted glycoside-based therapy for the treatment of hematogenous dissemination of tumor cells.