PROBLEM: Previous studies have demonstrated a requirement for RANTES (regulated on activated normal T-cell expressed, and secreted) at immune privileged sites; we have investigated the role of RANTES in the induction of maternal-fetal tolerance. METHOD OF STUDY: Endometrial and peripheral T lymphocytes were obtained from women with recurrent pregnancy losses (RPLs) and fertile women. RANTES modulation by progesterone or paternal alloantigens was measured by enzyme-linked immunosorbent assay or flow cytometry analysis. RESULTS: Progesterone significantly increased intracellular RANTES expression in CD4+ and CD8+ endometrial T cells. Moreover, alloreactive lymphocytes from RPL patients produced lower RANTES levels when compared with those from fertile women. At the local level, treatment with recombinant RANTES induced a decrease in CCR5 and CXCR4 messenger RNA that correlated with an increase in T-bet expression. RPL patients and normally fertile women express RANTES similarly, but differ in their patterns of RANTES receptor expression. CONCLUSION: RANTES may be implicated in the local induction of a Th1-type response necessary for successful implantation. Altered response to RANTES stimulation among some RPL patients may be responsible for poor pregnancy outcomes.