KIR3DL1 is a highly polymorphic killer cell Ig-like receptor gene with at least 23 alleles described, including its activating counterpart, KIR3DS1. Recently, the KIR3DS1 allele has been shown to slow progression to AIDS in individuals expressing HLA-Bw4 with isoleucine at position 80. However, due to the lack of a specific Ab, KIR3DS1 expression and function is not well characterized. In this study, we demonstrate KIR3DS1 expression on a substantial subset of peripheral natural killer cells through its recognition by the mAb Z27. The fidelity of this detection method was confirmed by analysis of KIR3DS1 transfectants and the identification of a novel KIR3DS1 null allele. Interestingly, KIR3DS1 is also expressed by a small proportion of CD56(+) T cells. We show that ligation of KIR3DS1 by Z27 leads to NK cell IFN-gamma production and degranulation as assessed by expression of CD107a. Furthermore, we document the persistence of KIR3DS1(+) NK cells in HIV-1 viremic patients. The high frequency of KIR3DS1 expression, along with its ability to activate NK cells, and its maintenance during HIV-1 viremia are consistent with the epidemiological data suggesting a critical role for this receptor in controlling HIV-1 pathogenesis.