We investigated the molecular species of sulfated sialyl Lewis X determinants, the putative L-selectin ligand, expressed on high endothelial venules (HEV) in human lymph nodes. Comparison of the reactivity pattern of HEV with the reactivity of the pure 6-sulfo, 6'-sulfo, or 6,6'-bissulfo sialyl Lewis X determinant with hitherto known anti-sialyl Lewis X antibodies strongly suggested 6-sulfo sialyl Lewis X to be the best candidate for the major sulfated sialyl Lewis X determinant on HEV, followed by 6,6'-bissulfo sialyl Lewis X, whereas 6'-sulfo sialyl Lewis X was unlikely. We newly generated monoclonal antibodies (mAbs) G152 and G72 directed against 6-sulfo sialyl Lewis X, which intensely labeled HEV in immunohistochemical examination and inhibited binding of recombinant L-selectin-IgG to HEV, suggesting that the determinant serves as a ligand for L-selectin. To test the concomitant expression of 6, 6'-bissulfo sialyl Lewis X, specific mAbs (G2706, G27011, G27037, and G27039) were generated, but all antibodies failed to react to HEV. Next, we established mAbs (AG97 and AG273) directed against 6-sulfo Lewis X, the asialo form of 6-sulfo sialyl Lewis X. The antibodies were not reactive to untreated HEV, but strongly reacted to sialidase-treated HEV. This indicated the predominance of the sialylated form of 6-sulfo sialyl Lewis X and minimal expression of its asialo form, corroborating that it was synthesized by fucosyltransferase VII, the isoenzyme that preferentially produces the sialylated form of the determinant.