A limitation in the ability to analyze the function of the natural killer cell Ly49 and other receptors is the lack of antibodies to most of them. Even when monoclonal antibodies (MAbs) have been generated, they do not always react with a given receptor from all strains of mice. For example, 5E6 binds to Ly49C and/or I from B6, C3H, and NZB strains, but fails to react with any NK cells of 129, C57BR, C58, or FVB mice. Besides Ly49 receptors, NKR-P1 receptor gene products (NKR-P1B) also contain an (immunoreceptor tyrosine-based inhibiting motif (ITIM) in the cytoplasmic domain (1 ). In the human system, the NKG2 family of receptor molecules with or without ITIMs associate as heterodimers with CD94 molecules (2 ), and are therefore candidates for murine non-Ly49 receptors for class I antigens. Molecules similar to human killer cell immunoglobulin-like receptors (KIRs) have not been cloned in mice. Other receptors receive activating or positive signals and therefore may have important NK cell functions (3 ,4 ). We describe here our approaches to develop anti-NK cell antibodies to study their receptors.