Objective: To identify an Fc receptor-like molecule in human cervical mucus.Design: Controlled experimental laboratory study.Setting: Department of Obstetrics and Gynecology, School of Medicine, University of Tokushima.Patient(s): Women undergoing treatment for infertility.Intervention(s): Sodium dodecyl sulfate-polyacrylimide gel electrophoresis and Western blot were used for analysis.Main Outcome Measure(s): A water-insoluble protein with immunoglobulin-binding activity was purified from human cervical mucus by ammonium sulfate fractionation. The initial 21 amino acids of the N-terminus of the immunoglobulin-binding protein were determined and analyzed in a computer search for homology.Result(s): The purified fraction contained a 15-kd protein that binds immunoglobulin A, immunoglobulin M, and all subclasses of human immunoglobulin G as determined by Western blot analysis. The amino acid sequence of the N-terminus is identical to that of secretory leukocyte protease inhibitor. The capacity of secretory leukocyte protease inhibitor to bind immunoglobulins was confirmed by Western blot analysis.Conclusion(s): A component in human cervical mucus capable of binding immunoglobulins was identified as secretory leukocyte protease inhibitor. The capacity to bind immunoglobulins is a unique property of the protein, providing additional support for the contention that it plays an important physiologic role in local tissue defense mechanisms. It also is involved in the pathogenesis of immunologic infertility by trapping sperm in the cervical mucus.