In this study, we carried out a phenotypic and functional characterization of lymphocytes isolated from the uterine endometrium of the pregnant rhesus monkey. A majority (80%) of these cells were CD56(bright+), CD3- had typical large granular lymphocyte/uterine natural killer (NK) cell morphology and contained numerous cytoplasmic granules. Flow cytometric evaluation showed that rhesus decidual CD56(bright+) cells shared other phenotypic features of human uterine NK cells, including low levels of CD45RA and CD62L expression. A majority of the rhesus uterine CD56(bright+) cells expressed low levels of CD 16 but were CD2-. In contrast, most rhesus CD16+ peripheral blood cells were CD56-. In addition to the primary population of CD56(bright+) cells, a minor subset of smaller and less granular CD56(intermediate+) decidual lymphocytes was identified, the majority of which were CD16-, CD2(+). Decidual CD56+ cells did not express monocyte/macrophage markers, including CD14, CD64 and CD68. Decidual lymphocytes effectively lysed K562, Raji and particularly 721.221 targets in cytotoxicity assays. Together, these results suggest that as in human pregnancy, rhesus decidual CD56(bright+) cells represent a distinct lymphocyte subset that belongs to the NK cell lineage.