Aberrant expression of cell surface carbohydrates such as sialyl Lewis X is associated with tumor formation and metastasis. In order to determine the roles of sialyl Lewis X in tumor metastasis, mouse melanoma B16-F1 cells were stably transfected with alpha1, 3-fucosyltransferase III to express sialyl Lewis X structures. The transfected B16-F1 cells, B16-FTIII, were separated by cell sorting into three different groups based on the expression levels of sialyl Lewis X. When these transfected cells were injected into tail veins of C57BL/6 mice, B16-FTIII.M cells expressing moderate amounts of sialyl Lewis X in poly-N-acetyllactosamines produced large numbers of lung tumor nodules. Surprisingly, B16-FTIII.H cells expressing the highest amount of sialyl Lewis X in shorter N-glycans died in lung blood vessels, producing as few lung nodules as B16-FTIII.N cells which lack sialyl Lewis X. In contrast, B16-FIII.H cells formed more tumors in beige mice and NK cell-depleted C57BL/6 mice than did B16-FTIII.M cells. B16-FTIII.H cells bound to E-selectin better than did B16-FTIII.M cells, but both cells grew at the same rate. These results indicate that excessive expression of sialyl Lewis X in tumor cells leads to rejection by NK cells rather than tumor formation facilitated by attachment to endothelial cells.