Inhibitory killer cell immunoglobulin receptors (KIR) bind to major histocompatibility complex antigens. Concise knowledge of KIR ligands allows prediction of natural killer (NK)-cell alloreactivity after hematopoietic stem cell transplantation. KIR3DL1 binds to the Bw4 epitope on HLA-B antigens. Although the same epitope is also found on 4 HLA-A antigens (HLA-A23/24/25/32), these are not currently regarded as KIR3DL1 ligands. We show that expression of HLA A*2301, A*2402, or A*3201 but not HLA A*2501 protects target cells from lysis by KIR3DL1(+) NK cells. KIR3DL1(+) NK cells from donors expressing the Bw4 epitope on an HLA-A antigen only are fully functional and capable of lysing Bw4(-) target cells. HLA A25 differs at amino acid 90, close to the serologic Bw4 epitope, from A23/24/32 and from Bw4(+) HLA-B antigens. These data suggest that HLA-A antigens should be taken into consideration when assessing the potential for NK alloreactivity after hematopoietic stem cell transplantation.