Objective: To study the effect of polyethylene glycated leukemia inhibitory factor (LIF) antagonist (PEGLA) in the human blastocyst viability and implantation process. Design: In vitro study. Setting: University hospital and research laboratory. Patient(s): Endometrial biopsy samples from fertile donors (n = 20), and surplus, frozen, good-quality human embryos obtained from an in vitro fertilization (IVF) clinic that survived thawing (n = 51). Intervention(s): Timed human endometrial biopsy on the day of luteinizing hormone peak + 4 days (LH + 4). Main Outcome Measure(s): Human embryo attachment rate, embryo quality, and expression of AKT and caspase-3. Result(s): PEGLA significantly reduced the embryo attachment rate to the endometrial construct. It decreased both mRNA and protein for LIF in the endometrial construct. Inhibition of embryonic LIF triggered apoptosis. Analysis of these blastocysts by immunofluorescence and real-time polymerase chain reaction showed a down-regulation in AKT activation and an increase in caspase-3 activation compared with the control group of blastocysts. Conclusion(s): The LIF inhibitor PEGLA could be a potential nonsteroidal fertility-regulating agent in humans. It acts on endometrial epithelial cells by down-regulating endometrial epithelial LIF. Inhibition of blastocyst LIF decreased its cell survival factor p-AKT and increased apoptosis (cleaved caspase-3). This highlights that embryonic LIF is vital for human embryo implantation. (C) 2013 by American Society for Reproductive Medicine.