The HRL1 monoclonal antibody specifically binds to L-selectin (LECAM-1, CD62L), which is detected on a small percentage of thymocytes and on most neutrophils and peripheral lymphocytes. CD62L is a 62-kDa (on neutrophils) or 65-kDa (on lymphocytes) receptor, with lectin-like and epidermal growth factor-like domains, which binds to sialylated oligosaccharide determinants on high endothelial venules (HEV) in peripheral lymph nodes. This member of the selectin adhesion molecule family appears to be required for lymphocyte homing to peripheral lymph nodes and to contribute to neutrophil emigration at inflammatory sites. L-selectin is rapidly shed from lymphocytes and neutrophils upon cell activation. In the mouse, the level of CD62L expression distinguishes naive CD4+T cells from effector/memory T helper cells. HRL1 antibody inhibits the ligand-binding activity of Lselectin in in vitro assays and slows in vivo leukocyte rolling on microvascular endothelium.