The monoclonal antibody YLI-90 reacts with the B6 allele of Ly49I, a member of the lectin-like homodimeric Ly49 family of cell surface receptors. Mouse Ly49 receptors exhibit allelic specificity for MHC class I Ia molecules and are thought to serve to prevent natural killer (NK) cells from attacking normal cells while allowing them to attack infected or transformed cells in which class I molecules have been down-regulated. These inhibitory receptors are also expressed on subpopulations of mouse CD8+ T cells. The YLI-90 antibody stains ~45% of NK1.1+CD3- splenocytes from C57BL/6 mice.