CD158e1 is a 70 kDa transmembrane glycoprotein and a killer cell immunoglobulin (Ig)-like receptor (KIR) that is expressed on a subset of natural killer cells and T cells. The monoclonal antibody DX9 is expressed on a subset of CD3-, CD56+ NK cells isolated from peripheral blood at varying levels (<0.1% to 61%) among individuals but not on thymocytes, B lymphocytes, granulocytes, or monocytes. Interactions with CD158e1 and certain MHC HLA-B antigens on a target cell inhibits cytotoxicity, possibly preventing target cell lysis and death. KIR and MHC class I interactions may regulate NK and T cell response to antigen stimulation.